Leviathan

I was teaching someone (male, can't remember who) how to do "magic" by using the mind ... if you could visualize something vividly enough, whether levitating yourself or an inanimate object, or making something come to you, you could make it happen. At one point, this person and I were on a raft of some sort in a body of water near a retaining wall when we were thrown from the raft by some force under the water. Panicked, I concentrated as hard as I could given my great fear, and shot straight up out of the water. A large creature came out of the water beneath me and was still coming up as my ascent slowed. I screamed in anticipation of being caught by the beast, and these screams woke me from the dream.

This dream was most likely inspired by Pirates of the Caribbean: Dead Man's Chest.