I just bought a Die Hard automatic battery charger today. The
instructions say that "a marine battery installed in a boat must be
removed and charged on shore."

The instructions indicate that it is okay to charge a car battery
while it is still in a vehicle. My boat is in my garage. I think
that it is okay to charge my boat batteries while they are still
hooked up to everything. It sounds like the instructions are meant to
not charge boat batteries while the boat is in the water.

I just want to be clear that I am not hurting anything by charging the
boat batteries while they are still hooked up.

Also, would it be best to use the 10 amp or the 2 amp charging mode?
It doesn't sound like I can just leave this charger hooked up all the
time, so I guess I should just use either mode and disconnect when
fully charged.