Google search the difference between wiring your batteries "parallel" vs. "in series". Heck - Here is a quick link to save you the search.
https://www.batterystuff.com/kb/arti...-tutorial.html
If you wire them up parallel, you could have 75 batteries in your boat if they would fit and it wouldn't hurt any of your electronics. Wiring nmultiple batteries together parallel doesn't increase your voltage. It's kind of like making 1 HUGE 12V battery if you wire it up that way. However, if you wire 2 or more 12V batteries in series, you'll damage all of your 12V electronics, including your trolling motor as each battery you add increases your overall voltage by an additional 12V. Putting 24,36,48 etc volts to your 12V electronics will ruin all of them.
Hope this helps!