Saltwater Suzi and Cap'n Larry's advice for the cash-starved boater
How to Books
Most boat owners love "How to" books.  The gift "How-to" book you select should depend on how handy he or she is and how willing he or she is to dive into a project as opposed to paying to have it done. 

Your boater may be good at electrical projects, or carpentry, or refinishing but might not want to get into fiberglass work or plumbing.

Talking to the spouse or significant other can help you in your gift selection.  If you are the spouse or significant other, you probably already know.  If not, ask him.  Better to ruin the surprise than get an inappropriate gift - at least that's our opinion.