I have had very good luck with Exide group 31s sold through Wal-Mart. These are their deep cycle maintenance free (sealed) as well as the serviceable type. I have two banks at my cabin
, both type 31s and type 27s. The 31s got a total discharge down to 10.5 volts last year and after an equalization all was good. My typical discharge is 20% at most 30%.
Another thought for a boat is an AGM (not gel - gel batteries can get gaps in the gel during charging that hurts them) battery if cost isn't an issue.
Proud owner of Stinkerbutt
-Air Raid cone filter, direct to TB
-Homebrew front air dam
-Homebrew side skirt
-Torza top bed cover
-Now featuring front wheel canards!