If you eat chalav stam there is plenty you can eat for breakfast and some things for lunch. Dinners are a notch above airline food but you can order a few per person. Some people have them wrap raw salmon or talapia a few times in foil and have them cook it (it's a common request and they are used to it) but not everyone is comfortable with that and you need ask your rabbi. There are plenty of fruits and vegetables too. I would suggest you ask for a fridge in your room and bring some salads, dressing and other other things to add on to the dinners and make it more enjoyable. You might also be able to brings some of your own frozen meals and have them store it and heat it up for you but I would suggest you get it authorized in advance.
All in all you will not go hungry and at the same time wont gain 10 lbs like most cruisers do. Unless money is not a problem it is definitely not worth paying 3 times as much for a kosher cruise. (in your case at least 10 times as much). I personally think cruises are the best value with very little planning.