There is no way you're going to waterproof an electric car to the extent it can survive total immersion in seawater without doubling the cost. Immersing an electric car in seawater will kill it. Immersing a gasoline burning car in seawater will kill it. Immersing a gasoline burning boat will probably destroy the engine too.

Dropping a car in the ocean is far, far outside of the design envelope. It is simply not reasonable to expect the manufacturer to add cost to endure that unlikely of an event.