This is a very common concern. I myself am very prone to motion sickness, so I have a lot of sympathy for this. It is very manageable. I have yet to have somebody come on a trip and be miserable all week because they were seasick.
A couple of things: first, your body will acclimate. Secondly, there’s a lot you can do on a sailboat to stay comfortable. Cruise ships, dive boats, whale watching boats, and deep-sea fishing boats are usually the worst for seasickness because they don’t adjust for wind or swell—they’re going somewhere no matter what.
With a sailboat, we stay flexible and sail in harmony with the elements rather than against them. From the skipper standpoint, I do a lot to keep things comfortable. My pro tip: if you want to get seasick, go below deck while the boat is moving. If you don’t, stay on deck, get fresh air, stay hydrated, and don’t overheat.
The captain will also give you a heads-up before sailing so you can take care of anything below deck beforehand.