Gentle ways to scrub bottom/waterline.

[Ryban]

I hauled out and re-did my bottom paint in October-November of last year. I used an ablative, ABC3, which I had heard good things about, and believe was the same bottom paint previously applied. I sanded most of the old paint away, applied a primer, and then applied the new paint.

Unfortunately, the paint has been all but useless. After a month in the water in Grenada, the bottom was covered with barnacles. Here in Bonaire the water is much nicer, but I'm still seeing fuzzy growth over the hull. Just above the water line, I also get that green, grassy algae attaching.

I'm trying to stay ahead of the growth, but at the same time, don't want to scrub away all my paint, especially at the water line, where I've already lost small patches. I try to be very gentle with a cloth, and scrub just above the water line, but it can be difficult to get the grassy algae off. Below the water, it's easy to scrub away the fuzzies, but I'm tempted to leave it be until I'm ready to leave, as I don't want to be repeatedly scrubbing away paint.

Are there any non-mechanical methods that won't damage the above-water paint, or the water line, that will help encourage the algae to detach?