there are lots and lots of cheap options in Cozumel, some oceanfront and some not. Go to websites like Expedia or Caradonna or BayAdventures for some, others you can get from websites like
Cozumel or check with some of the dive ops like
www.Papahogs.com or
www.aquasafari.com as they have connections with different hotels, but since you are looking at a last minute thing, be sure to check out airfare first.