If you want to stay stateside because of cost, the further down the Keys you go the better off you'll be. Not so much because of air temps, but because of wind.
Prevailing winds down there are from the East. The upper and middle keys--Key Largo (mile marker 113) to west of Islamorada (mile marker 80)--geographically are situated as east-west land masses. The lower keys, from Big Pine Key (mile marker 30) to Key West, geographically run north south. That means that you can tuck into the lee of the mangrove islands and hide from the wind--which ALWAYS blows in the keys.
You can wade Lower Matecombe west of Islamorada. It's easy to spot because everyplace else along the ocean (south) side is built up. There's a crabber's place on the right side of the road--lots of crab traps--and a wide open area on the ocean. That's a large flat that's all easy to wade. The Florida Bay side (north) is very mucky and nearly unwadable.
If you go to Ohio Key, park at the bridge and wade the big flat on the ocean. Check the tides, though. If the tide's out, so are the fish. You need incoming water, moving toward high tide, to expect any bones.
I don't usually guide down there unless it's a regular client who wants to do a week of bones. But Bruce Chard is good. Or simply call Christina at Sea Boots Outfitters on Big Pine or the names of some local guides.
Get back to me if you have any other questions that I might be able to help you with. Otherwise, have fun!!!
Oh, one other thing. Get a motel room ASAP. They don't come easy in the keys during the winter.