Re: Battery Sizing - am I crazy?
I would not use the battery that you linked to at Walmart. Not sure if it's a true deep cycle, and I would DEFINITELY not buy a maintenance free battery for a boat.
Get a deep cycle 12v battery, that you can maintain. Without going into the mechanics of how they achieve "maintenance free" status, I will keep this brief and tell you it's not what you want.
I would typically suggest a bank of two 6v golf cart batteries and a 12v starting battery for emergencies, but for the needs you stated, a conventional lead acid deep cycle battery will serve you well. Keep some distilled water on board and top off the battery regularly.