Yvon
If you merely need a starting battery on a boat, however, go
get a Sears battery for cheap. Don't try to run lights,
bilge pump, fresh water pump, GPS, radar, DC blender for
daquiris, etc. from it.
If you want a thorough read on this subject, get Nigel
Calder's boat maintenance book (2nd ed., "Boatowner's
Mechanical and Electrical Manual").
Good luck,
Rich Mechaber