Your biggest barrier to use will be the software, since most now have the no-fly-zones GPS programmed. I think the DJIs are all like that. Now assuming you beat that, you can probably get away with using it in Biscayne National Park. I've never seen NPS anywhere around there, and FWC is more interested in controlling all the drunk people on the sandbars than looking out for drones.
Staying completely legal, you don't have a ton of options, and you may be limited more by driving distance depending on where you're staying. The flats in Biscayne Bay are almost exclusively NPS territory. Everglades park boundary runs along the gulf side of the keys, but mostly with +/-1 mile buffer from land, until roughly Long Key. So until that point, you're limited to ocean side flats or the near-shore gulf side flats. Further south, you're fine anywhere. You can go north of ENP into some of the canals for peacock bass, too. Another option is the gulf park boundary isn't terribly far from Flamingo, and it's stone crab season, which means just outside of the park boundaries, you can find tripletail on the stone crab trap buoys. So weather dependent, you could always ask the guide to run to the traps, but to me, that's a bit of a waste if you only have one day booked.