I'll take a shot. Your battery is probably a group 24. It's a very common size for a starting battery. You'll want a marine starting battery which will have an 'M' designation. So what you're looking for is a 24M battery.
This is my starter battery for a 90 Merc. It'll be plenty for what you need.
Charge it after a trip and you shouldn't have to worry about running it down. An automatic charger or smart charger would be best.
Size 24M 800 CCA
https://www.interstatebatteries.com/...uctline=marine
Got it at Auto Value.
Remove it in the fall, store it someplace warm during winter and put it on the charger once a month to keep it in shape. I'm on year 4 with this battery.
You could probably get away with less battery (500 CCA), but without a lot of info I'd get the battery with the higher cranking amps. There's no harm in going bigger.
https://www.interstatebatteries.com/...eries/starting
Maybe someone else will chime in with another option.