Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Swimming in the sea indicates delving into knowledge or entering into the service of a ruler. If he swims and reaches the shore, he will get what he wants. If he does not reach it, he will remain in that matter. Swimming on one's back is repentance from sin.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Swimming indicates striving to fulfill a need. Swimming on land indicates prison and distress. Whoever sees himself swimming with another person is partnering with him in a matter or disputing with him.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Swimming is interpreted as salvation from hardships and fulfillment of needs. Swimming in clear water is lawful sustenance and useful knowledge. In murky water, it is worry and illness.