Swimming is an archetypal dream image. It suggests that spiritual energy is being used as we are moving towards a particular goal. It is said in dreams to symbolize the womb experience.
Psychological / Emotional Perspective:
To dream of being a good swimmer shows the ability to be able to handle emotional situations well, whereas being a poor swimmer in a dream could indicate the need to learn how to handle our emotions in a more positive way. Swimming in water will always be symbolic of the emotions, whereas swimming through the air connects with intellectual ability.
Everyday Material Aspects:
Dreaming of swimming has much the same symbolism as immersion. To be swimming upstream in a dream would indicate we are going against our own nature. Swimming fish can have the same symbolism as sperm and, therefore, can indicate the desire for a child. Swimming in clear water indicates being cleansed, whereas dark water could symbolize the possibility of depression.
Consult the entries for Drowning and Water for clarification.