You anchor at a reef dive site here in Belize and you'll end up in court! Either you use existing fixed moorings or you have a captain on board and the divers do a drifting entry. It isn't clear whether it's illegal to leave a boat unattended, but what is clear is that you are 100% responsible for the boat and anything it might affect. I can think of several people who have tied up their boat to a fixed mooring and returned at the end of the dive to find it gone. Either it became untied, or the fixed mooring broke (rare but does happen), or it was rolled over by a freak wave and sank (that has happened to several people I know).
You leave your boat unattended at your own great risk.
You leave your boat unattended at your own great risk.