Everywhere...they usually put the dive shops near the good sites.
When I was in Tahiti my favorite snorkeling spot was right off the room in the lagoon, because I could see neat things and then get out and shower right away, haha.
We honeymooned in Fiji and dove on the Rainbow Reef. Fantastic diving! We stayed at Qamea Resort. Also highly recommended (though that was 8 years ago)