Freedive fins & UWH
Vince,
Beside being dangerous to others, I don't think you could maneuver effectively in long blades to make them good for UWH.
I bought a new pair of Freefrogs on eBay for about $40 (including shipping) to use for snorkeling and freediving crevasses where I have to maneuver around rocks in tight spaces, and where thrust is not of supreme importance. They work really well for those purposes. The foot pocket is comfortable without neoprene socks, they seem durable, and they actually have a surprising amount of thrust. I've been to 19 meters with them in open water and felt that I had plenty of power to get back up.
I also have a pair of force fins that I never use since I bought my Picasso's and Freefrogs. Mine are the Pro Model Force Fin, not the even-more-expensive Excelerating Force Fin. The latter may be quite different from the former that I have, so I can't comment directly, but my guess is that the Freefrogs would be better for UWH. IMO, Force Fins are poor fins for surface finning because of the curved blade, and they are really geared toward efficiency during constant finning, not bursts of power here and there. Unless $$ are of no concern to you, I would try Freefrogs or the Avanti Quattro fins. Even without an eBay discount, you are looking at 1/3 - 1/4 the money.