When we went to Key West, I decided it was Bluewater Key or bust. BUT, either be a lottery winner or be willing to spend an obscene amount of money.
The first time we went was for Christmas 2014. We got a waterfront spot. It had our own private tiki hut and we did Christmas with my in-laws out in it. It was crazy expensive ($200/night with the extra people add-on fees) but still much less expensive than my wife's aunt and grandmother spent for a boutique hotel in Key West.
We went back for Thanksgiving in 2016. This time, we got an interior spot which had the most insane tiki hut. This thing was easily a 40' x 40' L-shape. It had a living room w/ TV, 10-12 person dining table w/ another TV, and a kitchen that had a full-size residential fridge, microwave, toaster oven, and sink. In addition to the tiki hut, the spot was extremely private and had a couple of sitting areas out and around.
On a budget, I've been trying to get into Sunshine Key and Fiesta Key through the Trails Collection program/add-on w/ Thousand Trails. But my travels are limited on the days I can travel and duration that I need to stay so we won't end up there until May :(. They end up being $20/night through the program. I've had many friends stay at them.
The state parks, if you can get in, are supposed to be beautiful. But you have to watch for cancellations and have flexible travel plans. Plus, I think length limits are in the 38-40' range (guessing! I could be wrong).
I've also considered Lazy Lakes RV as a budget option for Key West. It seems much more utilitarian than the luxurious Bluewater Key, but the price tag reflects it. We'll likely also try and give them a shot this summer.