Dang, I know that was going to happen!!
Ken, Been there, done that. The bilge in our boat is 44" deep and I dropped something down there years ago that I had to get back. A magnet wouldn't work, Sven wasn't there to offer his Shop-Vac so what was I to do? Several weeks ago you might have read the thread here in which butyl tape
http://www.filmtools.com/butyl.html was discussed. It's like bubble gum that never hardens and is good for bedding stuff. I was at a loss as to how to retrieve the object until I thought about that roll of butyl tape I had in the dock box wound around the end of my boat washing brush handle. If you can see the nut at all, do what I did but in your case, buy an appropriate length of small diameter wooden dowel or two, tape them together and buy one of these rolls (RV stores carry them for aluminum and glass sealing). Make a blob of the tape, stick it to the end of the dowel and go fishing. You will not believe how sticky that tape is. Simply lower it over the nut, press down for 5-10 seconds and that nut's there to stay. If everything you've tried including that fails, you'll have to remove the four screws that go through the cockpit sole that attach the base and get it that way. Even that isn't all that a big deal, after all you'll only have to tilt the base enough to grab the nut with some mechanical fingers or the like. At that point if it's ferrous, a small magnet on a telescoping handle will work perfectly inside the aluminum base. Be sure to rebed before reassembly. OK, so there's an alternative to add to the Shop-Vac trick. Don't keep us in suspense, curious minds want to know, tell us how you eventually get it out. Glyn Judson, E31 hull #55, Marina del Rey, CA