I have not done long distance foreign sailing; but I've taken my cat with me on my annual 2 week Chesapeake cruise.
The first several hours underway (especially if under power), she is miserable and seasick. It usually clears up by the time the first anchorage is reached or she pukes up her dinner, whichever comes first, at which point she becomes queen of the domain.
She is always under foot. She likes to walk the deck (scares the hell out of me; but an active cat is very difficult to contain down below). She has found a hiding place somewhere that I can't figure out, disappearing for hours and causing a fair amount of concern that I left her swimming somewhere back in my wake. A litter box is more convienient than twice daily dog walks, and provided it is scooped frequently, doesn't get too obnoxious smelling (every time I come below I check and scoop if necessary). Finding a good place for the litter box might be a challenge on some boats.
When coming dockside she wants to get off the boat and go wandering though. I spent a good forty minutes with the fuel
dock attendant in Oxford, Maryland chasing her down (the attendant was a fun gal and seemed to enjoy the hunt). But it clearly illustrates that Penny the Cat will not be contained if docked in a marina. I suspect other cats might want to go wandering, too. I'm not sure how other slip holders (or marina management) would feel about that.
I worry about her falling overboard (it's happened twice, once at anchor
where she managed to scramble up the swim ladder
before her splash registered in my brain, and once while docking when she made an ill timed leap for the pier), and will probably install new life line
netting to help protect against that, even though I seriously dislike the looks of netting.
There are challenges; but she is generally good crew and good company and will probably spend many more seasons with me on the water.