Why do scuba divers always fall backwards out of the boat?

Why do scuba divers always fall backwards out of the boat?
If they fell forwards they'd still be in the boat.

This is an elephant joke. The humor in the joke is the flawed logic that the jokester uses during the joke. The person hearing the joke assumes that the options are falling backwards out of the boat or falling forward out of the boat, but the jokester assumes that the options are falling backwards out of the boat or falling forward into the boat.

Since the unstated goal is to get out of the boat, the jokester is using a form of logic. It is just not the usual logic.