Trolling motors work with deep-cycle marine batteries, these actually being crucial items that need to be high-quality products to avoid being stranded in the middle of the lake. With this being said, you should also keep in mind that the indicator for battery life is ampere-hours.
Now, a battery that has a 1 Ah capacity means that it can deliver a 1-amp current for 1 hour. Smaller batteries range between 25 Ah and 50 Ah, while large ones can go as high as 100 Ah. of course, going for a battery that has a high Ah rating means that you can spend a longer time on the water.
There is no ideal number here, so the choice you are going to make depends on how much time you intend to spend fishing throughout the day, but you can take into account that an 80 Ah battery can last up to 20 hours.