Compared to petrol or diesel battery energy density is pathetic even taking into account the better efficiency of electric systems. Even pottering about at low speed a boat is energy hungry, a car will do say 50mpg a boat maybe 5mpg pottering. I can't remember how long he said it would run at 20mph but it wasn't far and if you press on will be a lot less. Remember the thirds rule, you need to get there and back with some reserve. Then there is the issue of charging. I don't see the point of these high powered electric engines, low power for displacement boats maybe.