I have been boating most of my life and have friends who have serious problems with sea sickness. This is the advice I give them.
1- watch what you eat the night before. Clam pizza is not the suggested fare. Try something that will not upset your stomach, something not acidic or spicy, not greasy and easy to digest. Moderate to low protein as protein is difficult to digest.
2- eat a breakfast but make sure it is not greasy, spicy, acidic and not too much protein. Just like dinner, except favor cereal and the like.
3- eat pretzels on the boat. I cannot emphasize the value of this enough. Pretzels are somewhat base (meaning they will neutralize acids) and with the bread content and salt they do a great job soaking up liquids in your stomach (which you do not want sloshing around while at sea).
Using these suggestions my friends have mostly conquered they problems and resort to meds only on the worst of days.