For a light load (especially if not using it while at rest) a cranking battery is cheaper and easier to find.
Closed cell? There are no closed cell marine batteries; rather there are "maintenance free" (if wet cell, actually a "less maintenance" in truth); AGM, non-spillable, but still can vent hydrogen; and Gel Cell, which require special charging circuits.
If you are a "normal" boater (i.e. don't get too crazy on the water), a regular flooded battery should be fine (use a battery box to trap acid should the case fail.) You may have to keep on eye on electrolyte level, but you'll save money.