Did something similar once, many years ago.
Pretty simple, ask the management of the marina for permission to post flyers, anywhere and everywhere they'll allow. If they are nice and OK with your plan ask for a list of boat ownwers w/addresses and mail them directly.
If really ambitious, be at the marina on weekends when owners arrive and hand them a brochure/flyer directly and ASK them for the job. You are always more likely to get what you want if you ask in person. Some people simply cannot reject someone trying to earn a living.
Maybe offer a "freebie" to show your stuff to those unsure.
Once you get started, word of mouth and other owners seeing you working will garner you more work than you want.