Cruise ship restaurants pretty much always have vegetarian options; low-salt, low-carb, gluten-free, and vegan food is either available and noted or can be prepared with advance notice.
Kosher food and Halal food must be pre-ordered before your cruise and are typically pre-made (depending on availability) off the ship and frozen for use during the cruise.
Kosher food needs to be ordered at least 30 days before sailing to have the order confirmed.
Halal food is offered on select cruises only, e.g. sailings from Dubai or Singapore.
In general, if you have a strict diet or a severe allergy, it's best to alert your cruise line in advance, either through your travel agent or a cruise line's special needs department. You can then meet with the head chef on the first day of your cruise to make sure you get the meals you need.
Please note: Special meal request is subject to availability on a specific sailing & is not guaranteed.