Another option is to make a 12.8v lifepo4 battery pack from the same website $23 per cell need 4 add a small bms to keep from over charge/discharge theres a 5% off coupon for that website somewhere just google it. Would be a good alternative. Would fit in a dry box and if taken care of should last you the rest of your fishing career. I have a 100ah im going to use for my trolling motor but thats overkill for just a finder
Would only advise buying from reputable sellers, most of the lifepo4 batterys on ebay are blems or used so they can be crap, but batteryspace is a good place. I have done a ton of reading up on these batterys so if anyone has any questions shoot me a pm and i can help you out. Yes they may cost a lil more than some NIMH batterys but lifepo4's have almost no voltage sag which i belive helps keep your finder accurate i belive. And if you take care of them should last 10k to 20k 80% discharge cycles, now idk about you but i dont think ill get in 10k more fishing trips in my lifetime let alone 20,000!!