OK Bear, but keep in mind the currents change on a hourly basis. One hour the currents can be mild. When there is the outgoing tide, they can be extreme. On the same windy day the outgoing tide can be dangerous but the incoming tide altogether different.This is where the knowledge of the guide is important. If you tried the idea of flags, to be accurate, it might necessitate changing them numerous times throughout the day which in my mind makes the idea unworkable.
I considered that before I made my suggestion bW. I think that in general there are going to be days when its simply not a good idea, based on the overall prevailing conditions, to allow people in the water; those are the days you target. You can get caught up in the minutiae of hour by hour conditions but from an enforcement and safety issue you need to, again, keep it simple.
To supplement Ragman's comments, the average wind speeds for that day and those hours (from the Mosquito Coast weather station, bless you Callum) were sustained between 15- 20 miles per hour. That's indicative to me of an overall condition. In fact Elbert's own morning observations well to the north indicated a tough day based on his own subjective opinion. I'm not saying Elbert is the say all know all in such matters, I'm simply saying a that guy who has been here a long time, with sufficient time on the water, felt there was enough empiric evidence from his observation to indicate a rough day. Currents may run strong or weak based on tides but the influence of wave action and wind affects those and are the components (imho) that perhaps made this day so perilous.
You'll notice that in Marty's post above his long time guide friend* mentioned coming back in over the reef, that's a bit tough to do with large wind driven waves constantly crashing over the reef ... you're not going to come go over that reef in a controlled fashion nor are you going to do so on your finger tips. Evidence that the reef was involved in at least one fatality is plain enough; whether before or after death, is yet to be determined.
*(whom I happen to think is a remarkable example and sets the standard for how a guide should prep his charges prior to Hol Chan)