From the parents point of view it must be horrible to have a passion for something and 2 children who sulk and moan the entire time and don't even make an effort to enjoy it!
I think they need to have a family discussion about it if they really hate it-(not something proved by the OP) but the DCs need to realise that parents are people too with passions, interests and hobbies and part of being a family is doing things that you wouldn't necessarily choose yourself for the sake of other members. If it was me I would come to a compromise, go slightly less, leave them with grandparents and go alone but they would have to accept that at 7 and 9 yrs they were coming sometimes- and if I was giving some of it up when they did come they had to smile and be pleasant!
Those people who deeply resent parents for dragging them around places had other problems in the relationship, I suspect,and were not generally listened to or consulted about anything.
I did lots of things as a DC that I wouldn't have chosen-being dragged off to motor races was the worst- but I didn't watch, I took a book and was quite happy. I very much suspect that the DCs have more time in Wales when they don't sail than when they do. It is up to them to use the time with what they want to do-take books, sketchbooks, board games etc.
The DCs are a bit pathetic if they can't amuse themselves. One of the best things that you can do for your DCs is let them be bored and use their imagination.
It is unfortunate that OP hasn't come back and given us more information. From the scant details we have, people have decided that the father is a high earning bully who gets his own way by continually telling them that he is paying for education so the trade off is spending all spare time on his hobby. I expect that it is far from the truth and that more than half the year is spent on child centred activities and, since private schools get far longer holidays than the father will manage, they have masses of time in the summer and spring when they are at home doing other things. When they are teenagers I doubt whether he will manage it (although they may love it and be grateful by then) but it is fair enough to take a 7 yr old and 9yr old with you.