The very act of swimming, immersing oneself in water for sheer pleasure, brings with it a likelihood of nakedness. Culturally, however, we are compelled to cover up. In this way, a bathing suit is a symbol of that modesty and the desire to keep a certain amount of protection. There is also an implication associated with this symbol of a connection to water that may not be present in the dream itself.
If you are wearing a bathing suit, then you have in the past or may in the future connect yourself to water, the ultimate symbol of emotional expression.