If you have a reasonable weight car, why not buy a cheap electric winch, rated up to 5 tons, stick it on the towhitch and just roll it out (with jockeywheel down!) That way you wont get your car stuck on that ever so slippery ramp-o-disaster.
IF you go for a tractor, might be an idea to get a 4wd one... as 2WD often does not provide enough traction on the algae, and requires a 2nd tractor to pull it up the ramp in the first place
4wd, diff lock on both axles, that would do the trick! I think a small "compact" tractor would actually do the job, however they are quite costly, especially if you only use them every now and then...
I think cost wise i'd settle for a tow hitch electrical winch.
8000lbs one should do, i think you can get a towhitch kit for it... and the rolling weight down the ramp of the boat should not be as much as the trailer +boat itself, unless its really steep.
Bleh, it's just an idea... it helps my post counts go up, doesn't it.