You can use an anti-fouling paint designed for fresh water but it will
still need to be cleaned once in a while. Best bet is to pull it out
on a trailer and pressure wash it, or skid it out with rollers and a
4wd truck.
I pulled the boat out of the river on to our floating dock ( wasn't sure I
could do that) last night. The algae is only along the bottom edge of the
transom - fairly harmless.