We preferred to sleep overnight at the jetty before sailing with the kids. The sleep on the yacht allows the inner ear and subconscious to relax and adjust to the movements.
Once a newbie has got use to the motion over a few cruises the seasickness tends to be less often.
I use to suffer terrible nausea when raising my hands up to reef the mainsail at the mast. The action combined with the boat motion set me off. Anyone can get seasick and its the experience people who are the worst when it hits them.
I make it clear to everyone aboard if someone gets sick, we will do everything to reduce their discomfort including heading home or to land as quickly as practical.
If someone shows signs of getting sick, we try and move them to a better part of the yacht for motion and change course to change the motion, it generally helps.
I never ignore the safety or entertainment of the crew, better to head back early than have a crew member in hospital with dehydration and all the worst stuff seasickness can cause.