I actually had the distinct pleasure of spending a week scraping sea life off of my boats screw. We could only use plastic scrapers, as anything harder could damage the screw itself, resulting in a noisier screw.
You must be really good at holding your breath. A week, huh?
I think I would have been tempted to use plastic-pellet "sand-blasting" on it. You control the force of impact by the air pressure.
It should have been quicker.