If "all" you want to do is lay on a beach, snorkel, have a few cocktails, perhaps golf and eat then an AI can be an alternative to cruising.
The bad part is that unless you have a car or do resort excursions that's about all you are going to do. There will be limited entertainment and my experience with a 4* AI (Moon Palace, Playa) is that the food is not close to what you can get on most cruises.
I would not rule out an AI but if you want to see more for less or equal money I would do the cruise.
BTW: There is a big difference between a 3* and 4* AI.