Absolutely... just ask your waiter and/or the M'd. The menu might even tell you which items are low fat/fat free. Stick with plain foods, in other words avoid sauces. Ask for mixed berries, even if they aren't on the menu, for desert. Get your salads without dressing or ask for a shrimp cocktail for each night's appetizer. You can ALWAYS get baked chicken and a baked potato... just ask. I know they often have fat free ice cream, too. Just don't be shy about telling them what you need. Your waiter will bend over backwards to help you.
Sorry you're not feeling well and hope you enjoy your cruise. On a positive note, bet you don't gain any weight on this vacation!