Re falling by the wayside on cruises, here are a few suggestions:
[1] don't go on cruises :-)
[2] Tell them you're diabetic and that you will turn blue and DIE if you have more than 3 grains of sugar! That'll scare them silly :-)
[3] "Party food is for parties" So pick a couple of occasions during the cruise when you can pig-out. Then follow your new lifestyle for the rest of the journey.
[4] Give up and start again when you get home :-(
[5] Check in to this forum every day and have a whinge, get some support.
[6] Jump overboard :-(
Avoiding sugar may be a problem in Asian ports. I guess sticking to fresh whole (unprocessed) food might help - if the salmonella doesn't get you! If they understand English tell them you want to avoid sugar and . . see [2] above.