Flying past a boat at "mast height" means if anything goes seriously wrong you have a couple of seconds before ditching. If he entered the water at speed this hints at CFIT.
An Engine failure would give you some time to convert speed to height, adopting gliding attitude with time to extend flap before ditching in a controlled, survivable fashion, possibly into wind.
Hopefully they are able to raise the wreckage.
Terribly sad thing to happen.