Good luck to you! I am leaving for on a 2-week trip to the Mediterranean on Thursday, 10 days of which are on a cruise...eeek! Italian & Greek food - my faves and all the cruise ship food...I'm a bit scared myself!
My tactics are: I bought a small travel scale. I'm not going to try and lose weight, but I am going to weigh myself every morning to keep my wits about me. Vacations really are a time for splurging, but I think stepping on that scale each day will keep me firmly in reality.
Also, I've resolved only to eat amazing food. If I take a bite of something and it's just "okay" - I'm not going to waste the calories. Same goes with the coffees and wines - if I don't love it, I'm not going to drink it. There will be too much amazing food & drink to waste calories on okay/mediocre food.
We also upgraded to a cabin that has as its main dining room a "healthier option" restaurant, not really "health food" - but smaller portion sizes (portions for one, not for 4!) - they use lighter sauces (no cream-bases) and offer a lot of seafood and fruit-based desserts.
I'm also going to continue writing down what I eat - again, just to stay in reality.
Also going to work hard not to eat mindlessly. You know, don't grab something and eat it just because it's there...eat at meals, eat a snack if I'm hungry, etc. Don't get that crepe covered in Nutella on the street corner and follow it up with gelato and a full-fat milk cafe latte just because I pass them out walking and it smells good. Hah!
We are also hitting the gym onboard during our at-sea days - and we have a packed schedule each day in ports and on our 4 days extension in Rome, so there'll be no "sitting around."
I think the keys for me will be: seek out smaller portions of healthier fare - only eat it if it's worth the calories - continue food journaling - be mindful of my weight every day - utilize whatever fitness facilities possible (including just WALKING) - be careful of mindless eating - HAVE FUN!