I think Wookie mentioned there was no US Coast Guard approved lift for sale that he was aware of.
I've seen him mention them before, but it has to do with a diver lift being considered an elevator, and the regulations for having an elevator on a boat are designed for actual personnel elevators like you might find on a cruise ship As such it is essentially impossible to have a diver lift.
I may be misrememberating, but it stands out in my mind.