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.
Diane, in every instance I can think of where red is displayed as a safety condition it means "NO", and closure.
My experience in regulatory development has been that simpler is better; the less interpretation the better, for example adding additonal restrictions such as the number of tour groups in the water etc. I just wouldnt complicate the issue.
No flag means no restrictions, Sure perhaps yellow flag to address your "difficult" conditions if you want, but I would never fly a red and allow people in the water. Red should be closure and whatever means works best for disseminating that information so be it. I would suggest a Hol CHan conditions phone number with a recording updated on a daily basis (or more frequent basis if needed). Everyone seems to have a cell and internet may or may not be available to the smaller guides.
With respect to the Rangers duties in such matters, if its closed its closed. It's easier for an enforcment officer to say "no" than " I would advise..." It leaves the officer open to having to provide explanantions, and perhaps subsequently "why didnt you just tell me, us, them, "no?" Believe me, as a guy who has been in such situations, I like it simple and direct. I'd bet good money the rangers at Hol Chan would approciate that as well.