Hi Barfe and welcome to Cruise Chat.
The regular menus and the buffets have lots of fish (non-shellfish) and vegetarian choices that should fit the bill. As Hank mentioned, you can order kosher meals but you'll do better ordering off the regular menu.
There are companies like Kosherica that specialize in kosher cruises, where they do serve kosher food on the whole cruise. You can search on the internet. I have no experience with Kosherica and do not endorse them in any way, but I have seen their ads.