To dream that you are underwater means that you feel out of some situation or relationships. Sometimes the dreamer who is underwater is trying to hide himself/herself from the world because he or she doesnt know how to act in certain situation. If you see the other person being underwater, it could have the same meaning if you were underwater and this person might need your help and support.
Dream dictionary: Underwater dream meanings
If you are swimming in a dream, such dream foretells about your emotions that are floating within. You are the person who does not initiate things by himself, instead you adapt to the present circumstances. The swimming is also a symbol of freedom. To swim underwater, means that you dispose other opinions and stay putted only with your own instincts.
If we dream that fishes are escaping from our hands when we try to get them out of the sea, then this reflects our emotional frustrations. Seeing fishes swimming between rocks or hiding underwater tells us about the desire of evading responsibilities that are a burden to us.