I agree with the suggestion that an advisory system for current strength should be in place at Hol Chan. There are no other local/popular dive sites over deep cuts - and it is the current in the cuts that creates the danger we're discussing.
Displaying a flag should be sufficient --
green for calm/novice swimmers are ok -
yellow for moderate/lifejackets optional/elderly and poor swimmers to stay close in
red / strong currents, lifejackets and ropes recommended - two dive guides per group
And - "closed" for the really really bad days. On "closed" days have Hol Chan rangers put out an internet message to the dive shops per a list of licensed operators - and have them stationed in the park telling errant operators that the place is closed. If the tourists hear it's closed/dangerous from the rangers they will probably take head and accept a nice exploration of coral gardens and shark-ray alley.
Inevitably we will still have some people going in the water (just like the passionate surfers who go out in a hurricane in Florida or wherever) - I don't think the rangers should arrest anybody - but they should clearly advise that this is an "at your own risk" and issue a citation to the operator for being there.
Then let it be - the divers will have been warned and will either take care or they won't - at that point they will be responsible for the outcome.