Electrical demands on modern boats are much higher than decade or more ago. Nearly all fishing boats have electrical devices such as lights, radios, sonars, refrigerators, laptops, electric trolling motors, electric watermakers etc.
There are several ways in which power sources can be classified, but basically they are shore based or onboard generators. Onboard generators can be fossil or bio fuel based internal combustion engines or generators based on renewable sources.
Although most boats can generate electric power from their inboard/outboard engines and use it immediately and/or charge battery bank(s), more and more boats carry auxiliary generator(s).
Also, carrying enough fuel to power main engines and electric generator(s) over a long fishing trip can be a problem for several reasons, thus many fishing boats are equipped with some sort of secondary generators such as solar panels, wind turbines, towed turbines etc. that make fishing boat electrically self-sufficient over long period of time, regardless of amount and/or condition of stored fossil fuel.
Note: one of the reasons to have secondary power source(s) are - emergencies. For example, your fuel gets contaminated with water (yes, I know, there are fuel filter systems that separate even water from fuel, but even they sometimes can fail!) making your main engine(s) useless, but also every other piece of equipment that run on that fuel - electric generator powered by separate internal combustion engine is useless, just as any other energy/power hungry equipment like trolling motors (can be used in emergencies to provide propulsion), watermakers/desalinators/water filters (do you know how much water during summer heat single person need?) etc.
Boat Electric Power Sources
Long story short, more or less standard boat electric power sources are given in the following list.
Mains chargers allow the batteries to be charged while the boat is in the port. Also, when in use, these chargers power entire boat's electric grid - lights, electronics, refrigerators etc.
Since larger boats can operate in different areas and in various countries, it is important that these chargers accept various voltages (110V, 220V, 380V), phases (single-phase or three-phase electric power), various sockets etc.
Good boat charger/controller is electronic device with several options (mains power, solar panel, wind turbine, etc.) for charging main battery (or battery pack), monitoring currents and voltages and sending alarms (audio, visual, SMS etc.) if certain thresholds are met. Read more about Chargers and Controllers.
Wind turbines harness wind energy. Blades are connected to the shaft that is connected either directly or over transmission box with electric generator.
They start to produce electric energy even during low wind speeds, but no wind, no electricity. Also, their blades spin and in strong winds, they can spin really fast - wind turbines must be carefully positioned on the boat in order to avoid possible damages or injuries. Read more about Wind Turbines.
Solar panels convert sunlight into electricity. Their power output depends on many things like sun radiation strength, angle of the solar panels, their type and age.
Unlike wind turbines, they have no moving parts and can be positioned more closely to the passengers and onboard equipment. They operate during the day, but during the night, they produce no electricity.
Very often, solar panels are combined with wind turbine(s). Read more about Solar Panels.
Towed generators, also known as hydro generators, are towed behind fishing boat under sails. If water is deep enough, it is good practice to submerge turbine generator deeper - up to to 15-20 m, for better efficiency. If you are on electric boat, or you are trolling using electric trolling motor, it is useless to use towed generator, since it will cause more drag (and hence increase electric consumption by electric drive) than it will produce electricity. Also, if you are anchored in strong sea current (in channels, for example), you can use 'towed' generator to produce electricity while you are anchored.
Some systems have the possibility to use rotation of main drive propeller to power electric generator, while ship is sailing. These systems are not very energy efficient, since propeller must be optimized for boat propulsion, otherwise such system can lead to increased fuel consumption. On the other hand, these systems are very handy and can charge batteries automatically - turn off main engine, gearbox to neutral (regarding main engine) and sails up, and you are producing enough energy to power lights, electronics and charge battery with few amps :)
Also, there is special type of generators I don't see very often - wave generators. They are positioned in the wake of the sailing ship or behind the stern of anchored ship. They harness wave energy, but some V-shape models harness both wave and sea currents energy. They are still in development, so, more on this some other time :)
Practically every internal combustion engine has some sort of alternator/generator. They produce energy when main engine(s) are running - to produce electricity, main engine(s) must run at least on idle, which is not always the best solution.
Some boats have main engine(s) coupled with generator(s) only - they are used to produced electricity (and maybe warm water), which is used to recharge batteries and drive separate electric motor(s) that are connected with propeller(s). This way, internal combustion engine (diesel, petrol/gas) always operate at optimum conditions leading to reduced fuel consumption. This system was even used on some aircraft carriers, battleships, submarines etc. and is still used on modern diesel-electric submarines, often combined with tanks of compressed or liquid oxygen and/or fuel cells - this system is found on modern German subs, Israeli subs, et. ... OK, if Germans are using them on subs, then it must be good ... :)
Fixed or portable diesel or gas generators are commonly found on medium or larger fishing boats when electricity is in demand. They have low fuel consumption, relatively low noise levels, portable generators can be easily repositioned, for example, on-shore when required, or transported for maintenance etc. Larger models with remote control and electric starters are preferred for onboard use on larger boats. Output power of fixed and portable generators ranges from 1kW to 10kW, even more. So does their price. Read more about Power Generators.
Manual power generators provide power to the most basic equipment in need - radios and radio beacons, cell and smartphones, navigation lights an similar. They provide tens of watts of power and they are not suitable for charging main boat battery - while providing 2-3 Amps, they would require 10 hours to charge the 12V battery with 20-30Ah. However, if you are unable to start the main engine manually due to weak battery, recharging it with just 5-10Ah is sometimes more than enough to bring the battery to life just enough to start the main engine - if that is the case, after starting the main engine, go to the nearest port and don't turn off your main engine until you are safe in the harbor (after which, you will find some good electrician to check your electric system). Read more on Manual Power Generators.
Fuel cells are new 'toys for big boys' :) They use fuel to produce electricity with efficiency reaching 70-80% easily - for comparison, the best diesel engines rarely can achieve 45% efficiency, with 35-40% being normal; petrol engines are even less efficient. However, fuel cells are still very expensive and if you run out of fuel, guess what - they don't work, no matter how expensive they are :)
Flow batteries are something totally new - personally I don't think that we will see them on the boats soon.
IMHO - it is vital to have good and reliable main engine with alternator that can recharge your batteries while running. To be sure, add smaller solar panel and if you have room on your boat one to two wind turbines. They are more than enough for most applications. After all, if you find yourself in emergency, you have a radio and some flashlights that are powered with internal batteries that can operate at least 4-8 hours without recharging, right?
Battery Cell Reversal
In order to achieve higher voltage, cells are connected in series - to create 12V battery, 6 lead-acid cells are connected in series. When under load, they provide electric current that is 'pushed' by the battery cells.
But, those 6 cells don't have the same capacity and if drained very low, some of the cells can 'push' the current through a fully discharged cell in the direction which tends to discharge it even further - cell reversal occurs - what was positive plate, becomes negative plate and vice versa. Cell reversal causes irreversible chemical reactions, resulting in permanent damage to the cell. And most often, when single cell is damaged, entire battery needs replacement.
Charging Currents and Risk of Explosion
When charging lead acid batteries be sure not to charge them with high currents - check manuals that came with your battery.
If you don't have such data, be sure to charge them slowly - normal charges that requires 10-24 hours are fine. Fat chargers that charge lead-acid batteries in 3-5 hours or even less are great IF and ONLY IF your battery was designed for such charging currents. Otherwise, excess hydrogen and oxygen can be created (gassing) and battery can bloat/boil and that can lead to cell/battery explosion. And nobody wants hot sulfuric acid and lead shrapnel flying around ...
If you have marine lead acid deep cycle battery connected to your engine alternator and have solar panel and a small wind turbine and you use your battery for powering lights, laptops, sonars, some small home appliance like small refrigerator etc, such conditions are really hard to occur. But, if you are using SLA battery for trolling only and you are constantly fully charging and discharging your battery, be sure to charge it with proper charger!