Not a marine biologist, but this seems like it would explain their entire findings. Yachts tend to hang out more around the tropics and less around murky waters in, say, the North Atlantic. Clear water, such as you'd find in the Bahamas, is clear precisely because of lower plankton populations[1]. By collecting data off of pleasure boats, they're corrupting the data right off the bat!
1: https://www.businessinsider.com/why-some-beaches-have-clear-...