Does your cat cope well with other forms of transport? -ie does he/she travel OK in a car or is travel sickness a problem?; will he/she curl up in a basket for hours on end? Does he/she like having people around him/her? Are your crew cat lovers? Is your cat a furniture and soft furnishing destroyer? What about scratching (they absolutely HAVE to scratch); is it used to a litter tray? are you used to the litter tray (no, i don't mean using it, i mean accidentally standing in it and then resisting the urge to kick the cat - which could turn into the temptation to throw the pooor moggy over the side) - and is he/she "programmed" or you might end up sharing a few fleas... and is he or she mature enough to not go chasing waves... (or get into a tight corner exploring in the depths of the bilge...)
You need to think about these points before taking your cat into any confined environment. Then start thinking about pet passport, etc...
If you are planningto bring the animal back to the UK at any stage you MUST have a pet passport or it will have to go into quarantine for six months. Even with the passport, there are so many ifs and buts it may prove difficult to bring it back without quarantine
There are many people with pet dogs and cats onboard in the med. My friends have two cats, Lucie and Duffy, and they live well. However there are some drawbacks such as; need to changing their toilet quite often and avoiding the quays. There is no problem putting a line stern to and keeping a distance from shore though. I also know a guy with FOUR dogs onboard. Pet passports can be a bit of a hassle. However, it is said that "if you like the rose, you take the thorns as well". Enjoy your cats and cruising, if you can take it.