I've got a little 1 amp charger that works well for me. It takes awhile to charge the (large) battery, but always seems to be finished before there's another chance to take the boat back out again.
I've got no battery switch, so always have to disconnect the battery cables after each use and flushout anyway. It's usually just go ahead and pull the battery, box and all, right out. It saved my tail once when my truck battery died, so is just kept in the truck as a spare when not in use.