From my limited experience kiting in the keys(~10 trips on atlantic side) I don't see foiling as an option without a boat. It just gets too shallow on low tide. We kited Curry during the eclipse and you could not even get back to home base without your fins on the TT scraping bottom hard enough to really concern you. That particular day, I can guarantee you there was no chance of foiling anywhere in the keys without a boat or walking a half mile out. I camped in Bahia for two days last summer and we kiting all over there and from what I remember unless you are a decent ways offshore you can't foil. It's pretty shallow, specially at low tide.
Othersideboardsports in Islamorada did a foil rental in February, which made me think they know what's up. Call them, they may have a secret spot, but most likely you need a boat or to go with them on a boat to get deeper water.
I've kited and boated Ohio Key (Sunshine Key trailer park) between Biahonda and 7 Mile bridge for many moons during the winter months. (Took my first kiteboard lesson from Paul Menta in Key West in 2001). On a NE wind the north side of Ohio Key and Biahonda have great depth and steady wind. Launching would either require a 50 yd shlog off the north side of the Ohio Key campground or snaking out through the east or west side channel. Haven't foiled there yet but I've kited all over the place and run my 24 ft center console to fish the bridges. Depths after that first 50 yards are fine. Of course the north side is not good with a SE-S-SW wind and you need to keep an eye out for lobster pot buoys. Note that there might not be much of a campground left after Monday