I use standard Halfords Car batteries.
The one I had in my old Flatacraft lasted 3 years worth of not being used/charged/maintained in any way for 6 - 7 months of the year, and was still fine when I sold the boat. It cost less than £30
It even survived being swamped several times and a complete inversion for about 10 mins.
The two I have in the current boat are now 2 years old, one 80AH and one 110AH, and together cost less than £100. They are Fine at the moment.
I share the load/charging on them by using the big one outbound and for starting the very cold engine, then switch to the small one for the return journey.
They are of course covered for 4 years, and Halfords don't know you're not using them in a car.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club