Because water symbolizes emotion.
In addition, water also represents a change, floods represent uncontrollable changes, and rivers and the like represent changes in life.
Being surrounded by water implies that feelings surround you and the land is getting smaller and smaller. This is reflected in emotional distress, and your down-to-earth feeling will be affected. It also reflects the fear of losing freedom and being dominated by feelings.
Swimming or playing in the water reflects the adaptation to changes in the environment and feelings.