Having been over large bodies of water multiple times, I can say this is pretty common. Honestly, knowing how to swim wouldn't help me, but it may work for you. My fear comes from the idea of having to wait to be rescued, not knowing what is under the water. My fear is that if the boat goes down, I could get tired and drown, or attacked by a shark, or it could be nighttime and I freeze, or anything else scary. I think a good idea would be to stay busy. There is so much to do on a cruise, staying busy may keep your mind occupied. If I start to focus on the fact that I'm on water then I start to get anxiety. If you do start to get some anxiety, maybe a couple drinks could calm your nerves.