Grand Cayman is the granddaddy of stingray snorkel spots, and sting ray swim operators are plentiful there.
I have done Sting Ray Swims in Grand Cayman, Antigua, and Nassau, but not the places you've mentioned.
BELIZE
Belize is not well known for sting ray swims... but I did find information that
About 30 minutes by boat from Belize City place nicknamed 'Shark Ray Alley.' You may need to book this with a private tour company if it is not offered by your cruiseline. Found this one:
Click here for details!
Experience Belize on your own and avoid the high prices and crowds of cruise ship sponsored shore excursions. With our tour, you will be able to snorkel two of the Caribbean's premier locations, relax amid the tropical tranquility of Caye Caulker, and save money all at the same time. Options for scuba diving, fishing, and discounted observer rates are available. Chips and salsa, plus bottled water, are included.
Price range: $29.00-$75.00.
As for the other places . . .
ROATAN
To my knowledge, there is not a stingray snorkel at Roatan, though the snorkeling there is supeerb, with the western barrier reef being within swimming distance of Roatan's beaches.. sometimes even wading distance. The water there is absolutely magical! There is, however, a Dolphin swim that we have heard good things about.
COZUMEL
We've done some great snorkeling off the coast of Cozumel, but no sting ray swims. The Snuba there is NOT something I would try a second time, but that's a personal opinion.