If you keep your boat around the Terra Verde area vs. St. Pete Marina, then you are getting much cleaner water running through your a/c system than farther inside Tampa Bay.
A couple of months ago we moved our boat from Marina Jack to Sarasota Yacht Club. You could almost see the growth happen overnight. (You could in a weeks time.) Major water flow due to Big Pass. That being said, I'll probably start acid cleaning the chillers twice a year instead of once a year. I add some bromine tablets to the strainer weekly. It cuts down on growth, but dosen't stop it. Part of the problem lies in the chiller and manifold design as well. With 5/8" tubing, there isn't much room for a restriction.
Lastly, my owners want the boat like a meat locker in the summertime. Colder the better, even when not on the boat. The more the chillers run, the more water you pump. Keep the boat thermostats higher to avoid having the growth foul you up so bad. Put blackout canvas on the windows, etc.