Thank you for your answer and advices
One of the crew member I was working with gave me 2 days ago the name and email of the man in charge of the recruitement at Costa! That's such a good news for me because I know that writing directly to the right person is way better
I'm actually working on my resume and letter, and I'll follow your tips about mentionning my experience on board.
As Xmas is coming, I plan to send it at the beginning of january. I really hope it's gonna work for me
Thanks again and I hope I'll be able to give you some "good news" soon!