In the early 1970’s, when I was about 12 years old, I was on a trip from Staniel Cay to George Town in the Exumas Bahamas. We were delivering a couple of C class sail boats for the Out Island regatta held annually in April in George Town Exuma. We had left on the tide in the pre-dawn hours and by early afternoon we had passed from the inside or banks side of the cays, to the outside or ocean side, somewhere south of Little Farmers Cay. The skies were clear and the winds fell off so that we were just drifting and sculling (these boats have no motor power). At some point in the afternoon, we entered an area where the waters were a rich deep purple color. And as we were not making much headway, we stayed in this purple water for quite some time. I asked the men who I was sailing with, men who had sailed all over the Bahamas, and their response was to the effect of thinking this as unremarkable. We were in waters that were at least 100 feet deep, off the shelf that runs along the ocean side of these cays. Under normal conditions, these waters are a deep blue which you can see down to 60 feet or more. To this day, and after sailing in many waters, including the Bahamas, I have never seen anything like that again. And my casual inquires have lead to no explanations either.
