2 or more problems Must exist. And it only happens in fresh water
More common is a lack of ac to dc ground bond. And then ac hot shorting to dc ground. Rubbed wire etc (Which puts 120v straight into water
. And breaker will not blow without ac-dc ground bond.
Nobody knows how often it happens . Because the cause of death from Coroner will simply be drowning.
Likly not very often. But does happen.
This is why every marina has a no swimming sign.
Iso transformers and elci’s make the boat