Congratulations...this is one of the most realistic terragen images I have ever seen. My only suggestions for improvement are: Waves tend to be more ordered than what you have, travelling in one direction or another, not so random. Second, your island is a bit too bumpy. Nice work overall.
I think its realistic because you don't see the land. The land is the hardest thing to make realistic, whereas the sky and water are easy to make realistic. I'm still having trouble making the textures look good on land forms. :)
I like the angel of the shot, but the sun at that angle should have much more red range. The artifacts in the water are pretty bad. Is the AA turned off on this render? Blue in the sky seems a bit generic. Also the island looks a bit like a cut-out. The hills are too defined, like there's only the set you see and no depth to the island behind it.
I hope you don't feel I'm too critical, but when you can custom order an island, ocean, and sun rise/set, I guess I expect a bit more.