Secure a fishing net to hang across the ceiling...put a few star fish, sea shells, drift wood etc in it.
If you leave one end long so it hangs down one wall or the corner of two walls a foot or two, it adds a nice touch too

If you frame your pictures with ship port holes, it will really set off the room.

Combine the above with a large salt water aquarium...and BAM, lol, you have a nice start to a beautiful sun room