I have been in Portofino many times in my 12 years as a private chef. It was my first destination on board a superyacht and I still love it. It is tiny, but breathtakingly beautiful. It is also eye-wateringly expensive. The chef in me gets a bit snobby because I always feel the food offered in most restaurants is average, but l guess it's about the whole experience. Also, average food in Italy is still pretty nice by any standards.
There is no rule book to being a chef on a yacht, at least there isn't one that works in every situation because, obviously, on each yacht you are working with different captains and different owners. If I am to offer any advice it would be to try and start with common sense and be prepared to learn as you go. Since I firmly believe in sourcing local produce whenever possible, when on Capri I use the fish, fruit and vegetables that are from the island and the surrounding sea.