His point is that it's not an islamic custom to bury people at sea, and he's completely right. The custom is to bury people in the ground, facing mecca, the same day of the death. That doesn't mean burial at sea never happens or isn't allowed or preferred under relatively rare circumstances, but an islamic custom it certainly is not. The custom would be to have buried him in the ground, not take him onto a ship and them dump him in the ocean, lashing him between planks to obviate bloating so that one may find him washed ashore and bury him properly. In short, he wasn't buried according to islamic custom at all.