If he was driving down the road, lost control, and plunged into the bay, yes, that is something insurance would cover. It’s an accident and you were operating your vehicle on a road, doing what you’re supposed to, and you fucked up. Maybe you were speeding, but in any event, you only ended up in an insurance “forbidden zone” like the middle of a lake because you left the highway against your choice. If they prove you were speeding they will raise your rates, and maybe drop you, but they have to pay for your damages.
When you drive down a boat ramp and in full control of your vehicle drive out onto the bay, insurance stops. The moment your front wheels leave the pavement of the boat ramp you have no insurance. It doesn’t matter if the lake has 3 feet of ice and is completely safe, but you find a warm spot and fall through, your insurance still isn’t going to cover you.
Same thing if you go to a track and flip your car. Sure, the flipping was an accident, but you were on a racetrack, which is another violation of most policies. In that case some policies have an exception to the exception in the case of it being a “driving school”.