No one wants to make things hard on the crew, but you have to take your cues from them as well.
When we cruised, my wife was abstaining from dairy and gluten. It wasn't hard for her to find choices on the regular menu for meals, but when it came to dessert, she was mostly stuck. She was going to go without dessert, but the waiter pressed her (gently). Did she like fruit, for instance? She said she did, and so he asked her what kinds. Over the course of the week they provided her with many varieties of fruit for dessert (sometimes whole, sometimes cut, depending on the fruit). Always enough for the whole table to try some. They encouraged her to ask for different kinds of fruit. If they didn't have it or if it was in short supply, they were honest about it (and polite) and she would pick something else.
They did it with such cheer and enthusiasm that I really don't think we were inconveniencing them. She felt special, and we all got some variety.
So I'd say be polite, be prepared if they say no or if they suggest an alternative, but by all means tell them how you are feeling and ask them if there are other options.
You may be pleasantly surprised!
