*Sigh* very romantic
IMHO The best place would be on the rear of the ship just as the sun is setting. You will have a full view of the ocean and the setting sun. Plus when it starts to get towards sunset most of the people are at dinner or at a show so you will have the private time you are looking for.
Good luck and please let us know how you finally do it. My BF could use a couple pointers
Just kidding, Carla