. . . I'm not so much hoping to see anything specific, but hoping to see anything new and interesting that might not be so commonly seen elsewhere. Is there anywhere you could recommend that might offer something that differs from the standand fare in either country?
How about whale sharks? Could be "new and interesting"--certainly not commonly seen ANYWHERE, as they can be elusive. They are found around Utila. Utila is said to be one of the few places in the world where they are seen year-round. Isla Holbox and Isla Mujeres, Mexico are similarly whale shark "hotspots." Keep in mind that trips to see whale sharks are done with snorkel gear, not scuba. You could spend a few days on whale shark outings, and a few days doing regular scuba diving.
When I think of sharks, the Bahamas comes to mind. Google something like "shark dive Bahamas."