May be a silly question but... I've cleaned houses for clients but now I've been asked to clean a ship so that it can be stored for winter. Anything that I should be aware of?

Barnacles!! And rust... Dive under the ship and clean all the barnacles off the bottom.  The rust you can take off with a wire brush, but be sure to paint the metal with a nautical paint. 2 or 3 coats. (be sure the metal is dry before painting)