The Seventh Stream (2001)

Five years after the death of his wife, Owen Quinn (Scott Glenn), a turn-of-the-century Irish fisherman, is still in a deep state of grief. One day, in accordance with Irish folklore, a seal emerges from the water and transforms into a gorgeous woman named Mairead (Saffron Burrows). Owen and Mairead meet and quickly begin to fall in love. However, the two realize their relationship might be short-lived, because, also according to legend, Mairead must soon return to the ocean and live as a seal.