Sugar, on the Chesapeake Bay it can vary by location, temperature, and even year to year. Last year was brutal, this year not much better. When bay water temps are in the 80’s as they are now, 2 weeks is pushing it for marine growth. What puzzles me is your term “ruining the bottom”. In my experiences marine growth can be removed and does not “ruin” the bottom, it is just how much work you want to put into the cleaning. Jerry