Hi, thanks for asking. The reef marks are large towers where there may be shallow water. Boats during the last race had no problems. Between those marks there is adequate water for catamarans, very unlikely to sail around reefs in this run. In the right conditions, the water is clear, like sailing over an acquarium, quite beautiful. One sailor said it was hard to keep his mind on the sails during a spinnaker run in the Islander Triangle race, because of the colorful reefs below his hulls. Study the charts of the area, most of the time the next tower is in view. wgg