This is a job we did on a BIG battery explosion.
The pic is how they found it. Good thing no-one was near it when
it went.
Root cause-Low electrolyte/neg lug rot, and damage to pos lug also. Got a big headspace full of hydrogen.
The spark came from a starter solenoid in the same compartment.
Opps..
GS:
Overcharging can also create enough energy to get the battery to go. Was it on charge when it happened?
There should not be any source of spark in a boat with an outboard. So something had to make it go.