Just looking for some advice on what kind of battery I should be getting.  I have a HB Piranha 170.  I don't know anything about marine batteries, so any help would be amazing!

If you are only running the fish finder a small 12V 12AH battery, or a motor cycle battery, will last you a few days fishing.

As far as brands the  upper end Walmart batteries (gold series or something like that) get good reviews and test reports show a decent battery. Another advantage is, if they go bad while you arte on a trip you can get a warrenty replacement just about anytime. Where do you go that a walmart is not fairly close anymore?

As far as the #1 rated batteries...Deka batteries. They a big bucks and rare. But none better.

My buddy is a freak about buying the best of whatever. He gets consumer reports researches for hours before buying anything. And this is his findings since he was getting new batteries last year.