Possible overloaded power strip charging everyone’s phones and cameras overnight. Very little to base this on but it makes sense to me. In any case this is a very sad event.
And how immediately the captain/owner said they’re not paying any claims.
Google: battery fires
Then click on Images.
But I agree that the most likely ignition cause is some combination of the boat's very limited electrical system with recharging of the many portable electronic consumer devices brought by the passengers.
I expect that underwriters of the coverage for dive boat operators will insist on an ironclad policy that all personal electric devices brought by the passengers be provided to the captain every evening that passengers stay overnight.