Need a new TM battery for a 12v motor. It gets heavy duty use, so I need it to be dependable and last on the water. Recommendations? Thanks.

lot of good options out there. I run super start deep cycles group 31 from oreilly's. They are made by eastern penn (deka). I just bought 2 last week for $92 a piece

 

Exide, Optima, Interstate are all good as well

I have had the same interstate going on its 3rd trouble free season.  Runs strong with all day use.  (Hopefully me posting this doesn't jinx me)

Get the biggest battery you can afford in a good, name brand. If it ain't ungodly heavy and will give you a hernia to carry it , it ain't big enough.

Walmart batteries have always worked well for me 

Marine grade size 31 deep cycle and you should be good for 5 yrs