A body of water such as an ocean, lake, etc. can represent:
You swimming or floating in a body of water can represent freedom from your usual limitations or stresses.
A body of water rising over your head, or you being pulled under water, can represent a feeling of overwhelm in real life.
A calm body of water can represent peace or relaxation.
Water that is flowing or changing shape can represent change in the dreamer's life—possibly out of the dreamer's control.