I don't really consider Kice/Romano to be part of the area I was describing. Yeah I guess it's in 10K islands. I've camped on that shoal out there, it's a shell covered wild beach, so that's kinda cool. More outdoorsy than beachy though IMO. I've been from Tiger Island down through Whitewater Bay, I've run Tin Can Alley and the outside to/from Islamorada many times. I wouldn't swim in there for two reasons:
Sharks
Fucking Sharks!
As for the west coach beaches I'm not a hater, but the OP asked for recs within an hour of Wellington. The other poster asked for east coast recommendations. So Cayo Costa didn't make the list.
As for crowding, Hutchinson Island beach access points will be less crowded at noon than pretty much any west coast beach. Once the flotillas arrive, it's just as crowded if not more so. As for cheek to cheek, east coast cheek >> old mid west, west coast cheek.