Oh I love sailing, you lucky thing. I am a day skipper. When are you going? Is it racing or just cruising? On the sea, or river? Where?
Jeans are fine, but something like combats/light �chino type� trousers are better. Basically anything that can dry out quickly � jeans have a tendency to get wet and thus chafing! But if you are going soon you need to remember it might be freezing out on the water, so go for warmth and comfort rather than any fashion consideration. Woolly jumpers ahoy. You shouldn�t really need wellies, go for cheapish light shoes like trainers or loafers which again are easier to dry out.
Wet weather gear is generally the final layer and should be a sailing jacket with a hood, and waterproof trousers � very fetching, but they do the job. With you on board there should be plenty of wind so enjoy it.