Me and buddy built one from a set of plans that I think we found in a popular mechanics publication (can't remember if it was the mag or one of their DIY books). Made from marine plywood, had sort of a canard wing for up down and and twin tailfin things for rudder control. was kinda cool, you could cover lots of bottom with it with very little effort. the entire thing was slightly positively bouyant so would surface if the tow vehicle stopped. Would probably work good over large sand flats or something with good vis.
In Travis, which in most cases drops off quickly and has rather low vis it was not that useful but was fun. We limited max depth by controling the amount of line payed out from boat. Did use it to scout out some new dive locations for interesting features, and once in searching for a guys grill lost overboard from his houseboat, but with < 20' vis it is only somewhat useful. speed is limited by vis in Travis, unless you just wanted a ride. You must also carefully watch you descent and ascent rates as it is very deceptive and you are going up and down quite a bit faster than you think you are and the effect increases with speed.
So, it is fun, but also could be very dangerous. We ended up adding a lock out to the limit of up down control, especially up, as when cruising along the desire to point assembly upward and breach like a whale is almost overwhelming.... and we have put on snorkel gear and removed our up/down lockouts so we could do just that.
