Given your apparent situation, I'd start with non-drug approaches to see if they work.
Prevention is a first step. Avoid fatigue and get adequate rest. Eat modestly, avoiding greasy, fatty, acidic and spicy foods. Don't skip breakfast, but stick to bland foods like toast, rolls and cereal. Go easy on tea and coffee, and very, very easy on alcoholic beverages. When on the boat, don't get overheated--stay under a sunshade, don't put on your protective suit until necessary, and get in the water as soon as possible. Search out a spot low and in the center where motion is minimized, face forward, focus your eyes on a fixed object on the horizon or elsewhere, avoid unnecessary neck movements and stay out of exhaust fumes. Do not go below, read anything or look through binoculars. If you must vomit, do so freely, but not anywhere on the boat, and especially not in the marine toilet.
Explore "natural" remedies, ginger is frequently mentioned, which can be taken in powdered, crystallized or root form (recommended preparations), or as ginger snaps, ginger ale, or tea. Just make sure they contain actual ginger and not just artificial flavoring. As heartburn with the use of ginger has been reported, also pack an antacid.
Many of the guidelines for the use of ginger in preventing nausea, vomiting & seasickness come from herbal lore, although one pretty solid study found 1 gram/1,000 mg of powdered ginger to do the trick.
I found sources that variously recommend anywhere from 1/2 gram/500 mg to as much as 4 grams/4,000 mg.
As with any drug or supplement, one is generally better off using the smallest effective dose. This may involve some trial & error.
Ginger in powdered, root or crystallized form is probably the easiest to control in terms of the amount of active ingredient ingested. Eating ginger snaps, or drinking ginger soda or tea makes control more difficult. And be aware that some products contain only ginger flavoring not real ginger--read the labels.
As for timing, estimates range from several hours to 20 minutes prior to departing, depending on the source. The matter really hasn't been studied sufficiently to say with any precision. The same issue pertains to frequency of dosage. The limited studies involving repeated doses used intervals of 4-6 hours.
Others swear by wrist straps, such as Sea Bands,
http://www.natlallergy.com/allergy/products/cart/search/view_one_spread.txt/spreadid/1220 usually wooden or plastic balls on elastic bands which are placed so as to exert pressure on an acupressure point on the inside of the wrist. There are also 'artificial-horizon' glasses
http://www.optic-tempest.com/textegb.html . Aromatherapy has been tried, with a combination of mandarin, peppermint, spearmint and lavender oil being recommended.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such.
Best of luck.
DocVikingo