I think you are talking about Seabob. It does about 7 knots on the surface and 6 knots underwater according to the marketing
Its a bit impractical for diving
. 3 knots is the threshold where your face mask begins to lift
off your face unless you have one hand on it. With swim goggles however you could really rip through the water
The last time I checked the price
for Seabob was about 8-11K depending on the model.
The more common dive scooters are about half that. I have been driving wet submersibles since 1976, both in the Navy
and now in my post military life. The biggest wetsub available today is over 30 feet long and runs on the surface like a boat then fills up with water
and becomes a sub.
Its always a spectacle when the boat is sinking or re-emerging to become a boat again.