This is Oceanous and Thetys. From Enc. Britt.: In Hesiod's Theogony, Oceanus was the son of Uranus (Heaven) and Gaea (Earth), the husband of the Titan Tethys, and father of 3,000 stream spirits and 3,000 ocean nymphs. In Homer's works he was the origin of the gods. As a common noun the word received almost the modern sense of ocean. Hence in details all sorts of "sea life" appear.

Sorry for the bad quality of some pictures, the museum managed to present almost any mosaic with spotlights producing glare, or with mixed natural and artificial light, the light colour changing all the time. This mosaic suffered more than the others.