As a child I went on the ferry to France (Plymouth-Roscoff) quite a few times and often felt a bit sick but this was OK after going out on deck and didn't put me off going on the ferry (and my brother who got more travel sick than me did actually puke a few times).
I've not been since I was a teen and planning to go next summer with my primary-school age kids. One gets a bit car-sick but is generally positive but the other is really worried about feeling sick (he's quite anxious) and if he has a horrible time I know he'll be stubborn about the return journey and will get really upset.
Do your kids get seasick, is there anything that reduces it and am I silly to worry about this? It is actually pretty awful feeling sick for several hours but I also think his anxiety might make it worse. On the other hand they might be so distracted by the novelty of it it might be fine! Any tips welcome...