I've been to Oahu, Maui, Kauai, Molokai and The Big Island, as well as the Bahamas, Bermuda, Aruba, Grand Cayman, Jamaica, St Thomas, St Maarten, St Kitts and Nevis so hopefully I can help.

The problem in recommending a Caribbean island that mimics Hawaii is that each Hawaiian island is so different from the other Hawaiian islands

If you are looking for a Caribbean island similar to Kauai, perhaps St. Kitts might fit your bill. It's not built up or touristy, has some beautiful views, good local flavor. The people could NOT be friendlier. The only 'name' hotel on the island is the Marriott. You can do a little snorkeling right off the Marriott beach. I didn't snorkel elsewhere on the island but St. Kitts is where the Caribbean meets the Atlantic Ocean so I am sure you could find other favorable snorkeling spots somewhere.

I've been to St. Maarten but that was only a day trip as a cruise ship passenger and, while the island was very pretty, I don't have enough knowledge of the entire island to compare it to Hawaii.

Grand Cayman was beautiful and has tons of great snorkeling, is not as commercial as the Bahamas or Puerto Rico, and has a small island feel. I would describe Grand Cayman as perhaps more commercial that Kauai but less commercial than Maui.

Jamaica has waterfalls and areas of the island are very pretty but the local feel may NOT be what you are looking for in a vacation and would not remind you of what you've probably experienced in Hawaii.

Bermuda, while not technically in the Caribbean, is very pretty and has a small island feel; beautiful beaches as well....and may be closer for you in terms of travel.

Hope that helps a bit