I'm almost 2 years post op from gastric sleeve (8/10/20) and will be going on my very first cruise in December. While I haven't been on a cruise yet, hopefully I can provide some insight in regards to WLS.
At 8-9 months PO you'll be able to eat anything you want (with discretion if you know certain foods end up affecting you when eaten), and depending on your doctor's plan, be able to drink alcohol. Alcohol hits faster and harder with our small stomachs though.
I've read people were able to order the steak from the kids menu for a smaller portion, and that's what I totally plan on doing, but then also just ordering what I want. Thankfully I have a skinny fat boyfriend who will eat the leftovers, but also don't feel too horrible about food waste since it tends to happen anyways.