Note: we have really tried to verify every single data in this table, but ...
There are many other battery models and manufacturers on the market, these are most common and most popular.
Which battery to choose? Choose according to your storage area and power/energy requirement. If you are not sure about it, but have room to accommodate 31 size battery and money to pay it, go for Exide XMC-31 or VMAXTANKS SLR125. Also, these batteries are quite heavy and when positioned low in the small boat, they lead to much improved stability. Position them high and it is accident waiting to happen.
Although classified as 'Deep Cycle', Exide XMC-31 (improved Exide MC-31), with MCA rating of 1000 Amps is nice 'Dual Purpose' marine battery. If you don't require large starting current, then XMC-31 can be used as 'Dual Purpose'. If you do require larger starting currents AND good trolling motor battery, consider Optima D31M.
Warning - some manufacturers prefer their batteries to be charged with chargers optimized for AGM lead acid batteries and they even forbid charging them with chargers designed for flooded cells lead acid batteries - what ever you do, be sure to read the manuals! Don't lose warranty for batteries that can cost hundreds of dollars/euros because of a cheap charger!
Trolling Batteries - Little Bit of Math
You need 12V battery with at least 200Ah capacity that is capable of providing 1000A of start current, but you are unable to find it locally?
However, you can buy good gel cell 12v/110Ah/800A batteries. Are they good enough? Well, they are good, since connecting them in parallel, you will get a battery pack that has 12V voltage, 220Ah capacity and is capable for 1600A for starting (btw, 1000A is HUGE current and 1600A is ... uffff!!! :o) - this is an example, but ...). Due to slight differences in voltages among lead-acid battery types, be sure to buy exactly the same batteries from the same manufacturer and preferably from the same batch - when starting your engine(s), you don't want one battery to provide 700A and another battery to provide 300A! Also, be sure to connected them with properly sized cables and individual high-current switches and (automatic) fuses.
Keep those batteries away from heat sources, keep them apart at least 5-10 cm (2-4 inches) so that sides can be cooled by air, too.
If possible, attach temperature sensors to both of the batteries and monitor their temperature just as if you monitor temperature of your engine (cooling water, oil etc).
You need 48V/120Ah/200A battery (voltage, capacity, starting current), but you can buy only 12V/110Ah/800A battery.
Well, sorry to say, but you need 8 such batteries. First, connect four pairs of batteries in parallel and then connect those pairs in series. This will create a battery pack that has 48V/220Ah/1600A - this can cost a lot and cost of professional services are just a fraction of such costs.
Or, create 2 groups of 4 batteries connected in series and then connect them in parallel. This second option is usually preferred one, since one can use batteries in groups of four same batteries, but they can vary from group to group (various capacities from group to group, but the same capacities in the group and the same battery types in whole battery pack).
Again, such battery pack is very strong - it can provide 4 kW for about 2 hours (batteries drained to 20%) or ~77 kW for 5-20 seconds without any cell damage!
Since we have batteries connected in series, there is a danger of cell reversal - be sure not to drain such packs bellow 20% of their capacity, just to be sure.
You have SLA 12V/110Ah/800A battery and 12V trolling motor that requires 14A at maximum power and 8A continuously. You don't plan on using it on maximum power, except for short periods (10-15s - which is negligible).
You want to know, for how long you can use this battery safely.
First, 110Ah capacity is actual capacity when battery is drained for 20h at little more than 5A; but we need 8A, so we don't have 110Ah. If you have drain tables of your battery model, be sure to check capacity of your battery when drained at 8A - in such tables one can also find for how long that battery can be discharged at 8A to 10-20-30-40 etc percentage of its capacity.
If you don't have such tables, instead 110Ah, consider that you have 100Ah battery and when discharged to 20% of its capacity, we have 80Ah at our disposal for our 8A trolling motor. 80 divided by 8 is around 10 hours of continuous trolling with fully charged battery.
Note that if you have older battery and/or older model of battery, you can't expect to have 100Ah at 8A, so instead of 10 hours of trolling, you should expect less - maybe 8 hours, but this again depends on state of your battery, temperature, peek usage etc.
This calculation provides you with information what you can expect from your battery/trolling motor combination, but for real, accurate data, you will have to test the battery with 8A load and monitor temperature and voltage of the battery.
Deep cycle trolling batteries will do their job well, if used properly. Don't misuse or abuse your battery, or they will abandon you when you need them most ... 'Murphy's Laws' at their finest!