Hi DDW,
I'm surprised more people with a divided underbody don't do this... I have small attachment points on the trailing edge of my keel, and the leading edge of the bottom of my rudder, to which I clip a loop of 1/8" Spectra line... After the boat is re-launched with one end already attached to the keel, I simply dive quickly to the with the other end in hand, and clip it onto the rudder... Hell, I could even do that in Maine... (grin)
I finally configured this solution after snagging a pot off the Florida Keys years ago, haven't caught anything since... Not foolproff by any means, but it sure seems better than nothing. When it's time to haul the boat again, if it's too cold to dive to unclip it, I could simply saw it with a hook knife, or similar... I consider a hook knife essential gear, btw, I wouldn't cruise anywhere without one, especially in cold waters...
All boats are compromises, of course, and my old IOR design certainly has its share of drawbacks... but, one of the features I like best about my boat is the position of my prop, and the relative ease with which I can reach it from the cockpit with a hook knife on a pole, or from the water's surface... Makes it so much easier to "bag" the prop with a garbage bag when she's gonna be sitting at the dock for awhile, as well - which I have found to be far and away the best deterrent to prop fouling... I've come to consider such access to the prop a pretty important design consideration for me, it I ever own another boat, I would want to have a similar configuration...
One of the most dangerous situations I have ever dealt with offshore, was having to untangle a mass of fishing net we had wrapped on a delivery to the BVIs a couple of years ago... Roughly halfway between Hatteras and Tortola on a Trintella 50, with the prop underneath a relatively broad stern hull section, no way to reach it from deck... We had been motorsailing during the only spell of light air we had during that trip, the swell had moderated considerably, but diving to free that net was one of the scariest things I have ever done at sea... I'm one of those who is basically terrified of going into the water in the open ocean, I revert to a little kid imagining a monster under my bed... (grin) I only did a small portion of the work, fortunately there were two other experienced divers aboard, we worked together in shifts. I was amazed how much effort had to be expended simply to get to the prop, before one could secure yourself to begin the chore of cutting the mess away... Had I been singlehanding, no way would I have been able to deal with that situation at sea...