Well, for which is the most fun - Ocean out on the Great Barrier Reef. Well, any coral reef really, that one just happens to be the closest.
However, at the same time I absolutely hate not being able to see everything in the water - I am constantly pessimistic that some shark or other bugger is going to come up and eat me, and when I have to swim about 50 meters over open, seemingly deep [But probably only 20-50 meter deep] water that I can't see the bottom of I get a little nervous.
However the outer reef was brilliant. About 10 meter deep water that I could see the bottom of, on a floating... I dunno what you'd call it but it was basically a base of operations on the water, but not a boat, with a water slide and awesome massages. The water there wasn't too cold either - which is my other main concern when swimming. Get me in a normal ocean at about 15-20 degrees and I will be purple to blue within 30 minutes, even when wearing a wetsuit. I just freeze fast.
Hence, for comfort swimming, a heated indoor pool is where I'd swim. Preferably 35 Celcius or above. Even at 30 degrees I'll only last a couple of hours before my teeth start to chatter uncontrollably.