Just to note, after charging be sure to ventilate the boat prior to operation.
I had a friend that put a brand new 6v 4ahr battery into his submarine.
He sealed it up in his water tight compartment, charged it up and took it to the pond.
The submarine all set up, he set sail, made a turn and commenced a dive.
Upon operation of the ballast pump the water tight chamber exploded.
Not much damage.
He repaired everything and subsequently repeated the operation above.
Turned out that the battery was venting hydrogen during the charge, and his
ballast pump motors were "sparky". Now he has a vent hole that is open during charging, and
he runs a vacuum on the hole before sealing up the WTC for sailing.
Be sure to ventilate the model before sailing if your are charging batteries in the boat.