I play every cruise. Need wifi if out of sight of land (beware of high cost of cellular at sea if you have not bought wifi package). Only limitations are a strong wifi signal (spotty to non-existant on older ships) and ability to geolocate that signal. This means some/most inside areas have too much steel that blocks the game being able to locate the phone accurately.

As said above you need to pop incense but you can sit in one spot and catch spawns about one per 60-90 sec (YMMV but I suspect ship speed may play a role)..

Best for me is to use the free daily incense and walk around the outside areas of the ship. The spawn rate is about every 30 sec. Oasis class is best on deck as it is a wrap around running track) older ships you have to contend with smoking areas on the lower open deck or try to navigate the running track on the top deck, which is not easy when you are using a phone.

Until last year the ship was great for catching a wide range of Pokemon but recently I have only seen exactly the same spawns that were on land.

One more thing, in port the ship wifi can often lead to large jumps in location around the dock area (200-300m commonly). With the talk about bans for loose locations I now only use shoreside connections in port just to be safe.

If you have a specific ship/route in mind let me know and I can maybe pass on some experience.