My fear of the ocean, rational or irrational?

I am terrified of the ocean. When I have a nightmare it is usually of me floating in the middle of the ocean. There isint a storm going on, it's clear skies, no sharks about to attack me. I'm just floating there. But when I think about I'm always scared. I have never had a bad experience at the ocean. I went to cape cod when I was little. I went into the ocean and the only bad part were all the shells on the bottom. But when I think of that dream it sends chills down my back and I'm truly scared. I feel that I could never go on a cruise because of that nightmare. Why do I keep having that dream and what does it mean? Please help! Best answer gets 10 (glorious) points (lol).