Seems to me that it would be quite scary coming to the surface at high speeds in the thing, and what about the hazards below? Smacking into the hulls of other ships, getting tangled in anchor rodes, eaten by sharks.
It only very quickly dips under the water
. It doesn't go down far or for very long. You woudl never be able to "pop up" under some hull
or anything, unless you purposely tried to dip down directly infont of one. As I said, I have seen guy's on jet ski's do exactly the same thing. It was that very trick on a PWC that gave the original designer
the idea. Gee it must have been 10 or 15yrs ago now. I could be very wrong of course, but it seems such a long time back.