Have I got a site for you, wwh:

http://members.aol.com/arthuswint/terms.html#C

This site provides a glossary of nautical terms among which is included careening, and apparently careening was done specifically for the purpose of removing barnacles. There's no mention of any speciic tool that was used. [2nd Edit: Another glossary on another site mentions careening for scraping barnacles and ship repairs.]

I vaguely remember your story--and I remember that there was a prank involved?

1st Edit: Apparently the photograph links on the site have timed-out, but the definitions are still there.

3rd Edit: Just found a figurine online of an old salt scraping barnacles off his little boat with a brush and a chisel.