We noticed when we were traveling up and down the California coast that we'd quickly get a little green grass beard (1" or so long) along the waterline. After getting into San Francisco Bay on the way North, we cleaned it off and wondered how it could have fouled MORE while we were moving a couple hundred miles than when we were just puttering about in So. Cal. Then, we spent months in the Bay area with no growth! We cleaned the hull right before coming down the coast but nothing was growing there. Spiffy clean. Three weeks of fun sailing all over the place before pulling into San Diego...and...there's a little beard of green at the waterline again! So, we're thinking "slow travel, no growth, fast moving water, lots of growth"...hummm...I'd had a biologist tell me that some organisms have evolved to latch onto rocks (or boats) when the water is moving fast--so they'll get nutrients. Makes sense.
I just ran across an article that discusses this "fast water and things grow idea"