One of my favourite swimming - well ANYTHING - spots in the world is Nacula Island in Fiji. Nacula (pronounced Nar-thoo-la) is a 4 hour catmaran ride away from Nadi on the "mainland", in a group of islands called the Yasawas.
First, there's the beach at the Blue Lagoon Beach Resort - I've stayed there three times so far: August 2013, May 2014 and May 2015 (these photos are really truly my photos of what it's like - nothing doctored!)
And if that beach isn't enough, there's more a little way around the point (a good place for some beach Pilates!):
And often it's what's underwater that makes for the best views:
And if none of that suits, there's always the pool back at basecamp!