I want the battery that will last the longest. I know nothing about batteries. Can anybody point me in the right direction?

For running a trolling motor or a sonar unit? You need to determine how many amps either will draw per hour, look in the tech specs of whatever you're running.

if cost is not a problem... liPo batteries - do NOT go cheap or you risk fires like samsung... LIPO batteries are great... but you need to have the right safeguards in place and cheaper batteris of the type skimp on those.

biggest one i could find in 60 seconds of searching on amazon from a reputable brand 102watt hours - https://www.amazon.com/dp/B00F9LPL5E?ref=emc_b_5_t

If your looking for something for a fish finder - look in the rigging forum - i posted two builds i did last year that should meet your needs, are safe, and have tons of options right up front.

Deep cycle and match the size of the battery (Amp hrs) to your load and run time. You can get 100 Amp Hr and bigger.
Lithium ion will give you the most duty cycles but the are $$$ but very light weight.
There can be a lot of difference in battery brands and a lot are made by the same battery manufacturing
Company but under different names. Do your research there's a lot of bad. Formation going around about batteries.

Battery Tender has a affordable lit ion and if you want to go bigger look at Stark Power. I'm running their
30 Amp Hr and love it, $$$

I bought a 100 amp hour battery off Amazon and it powers my TM on my kayak for at least 14 miles or 4-5 hours of continued use on full power. I still have charge left when I recover. A bit heavy at 60 lbs but I like to stay on the water as long as I can.
Amp hours is the best way to determine the size you need. There is info on the web on how to calculate the amphour runtime of your TM.

The lithium ion weighs about 28 lbs but it's $$$$