a plastic windscreen scrapper gets them off, you want to do it when that are still alive and straight out of the water. If not they leave a ring thats rough and tough to scrape off afterwards.
PS Photo's are too small to say anymore
good luck
Jelly
AS already said, leave to dry out and flick 'em off. Not sure if they do damage to tubes though?
Check the engine hasn't been left in the water as well though as you'd be amazed at the little creatures that like to crawl inside any hole they can find.
