On AC, we've seen lots of eagle rays on each snorkel trip that we've gone on. It seems to be a very good possibility that you'll see them, and often you're in ~8ft of water so you really feel like you are swimming with them.
On the Big Island of Hawaii, we did a Manta ray night dive and swam with at least 6 Mantas. One had an 18ft wingspan. The boats set up a light at the bottom of the ocean, and the rays come to feed. It was amazing, having all those huge rays playing around us. The divers sit on the bottom and try to stay out of the way, but many of them got bumped pretty hard as the Mantas looped and swirled.