I've recently spent a week out in the South Pacific on Lord Howe Island. It's about 600 km off the mid-north coast of New South Wales, and has the most southerly coral reef in the world. This world-heritage area has a relatively shallow lagoon with several beaches, and also has ocean beaches on the opposite coast. Plenty of swimming spots to choose from.
Mount Gower and Mount Lidgbird seen across the lagoon
The water is incredibly clear. There 90 species of coral and 500 species of fish from both the tropics and the temperate regions.
Beach at Lovers Bay
It was lots of fun to swim, with mask and snorkel, from beaches along the lagoon, out to reefs of gorgeous coral and algae. At each reef, there were many colourful fish, both large and small. It was easy to spot parrotfish, angel fish, anemone fish and so many more. Off Lovers Bay, I swam next to a large turtle.
View across the lagoon to North Bay
From North Bay, I swam out to and old wreck that was filled with shoals of catfish and butterfly fish.
Sunrise at Neds Beach
At Neds Beach, on the ocean coast, the reefs are very close to the shore. There's a sand channel that can be followed out to deeper water. I saw turtles here too, and also swam with three (small) Galapagos Sharks nearly every morning.