We leave our boat unattended all the time including in Florida while diving, it is not unattended, we are below it. I have also tethered off to my kayak nearly every time I have used it. Note, without knowledge of local conditions it would be very unwise to leave your boat without somebody to come get you, new to the area for diving and boating is asking for trouble. The Keys and the northern Gulf can both have strong currents that easily can sweep you away if you get lost on the bottom and surface down stream.
The buoys are public and are for the public. It is good etiquette to use them in a timely fashion and then move off so another boat can make to them, in other words share.
However, for example, the Mighty O the buoys are actually submerged, you have to send a diver down to hook on. The buoys on most Keys locations are on the surface. In the northern Gulf it is routine to set anchor, in the Keys and other areas with living coral, that is a bad thing obviously. However, the O is sitting in about 240 feet of water, anchoring, at least for my small boat, is not really practical at that depth.
To the OP, when you say "small" sailboat I have no idea what you mean, sailboats generally make horrid platforms for diving and if there is a fixed keel many of the passes in the Gulf and shoals and reefs in the Keys are challenging to navigate as it is without a keel dragging bottom. Beware, running up on a reef will ruin your day and now that might result in a fine if coral is damaged. But of course, boats have been running aground in the Keys for hundreds of years.
Four foot seas might be rough but we have came in with much worse over the years in small boats, next, what, we have marine patrol measuring the average wave height and issuing tickets for negligence.
N