I encountered the prefix while reading an archaeology magazine article about the ancient Minoans, who were referred to as having a "thalassocracy." The meaning was evident from its context in the paragraph, but I wondered re the etymology and the relation to the more familiar "thalassemia" (a disorder of red blood cells). A quick browse through a couple dictionaries sorted it all out.
Thalass(o) is derived directly from the Greek term for "marine" or "sea." Thalassocracy is thus mastery of (or sovereignty over) the sea and thalassemia is a genetically acquire disorder particularly common among peoples whose origins are from the Mediterranean Sea region.
Some other related words:
Thalassic/thalassian = pertaining to the seas (esp. smaller bodies of water rather than oceans).
Thalassian = a marine turtle.
Thalassa (the root word for the prefix) was a sea goddess (and the personification of the Mediterranean), and is the name given to one of the moons of Neptune.
Thalassography = oceanographic science as related to bays, sounds, gulfs, etc.
Thalassin = a poison in the tentacles of sea anemones.
Thalassinian = related to decapod crustaceans (scorpion-lobsters).
Thalassarche = the family name for albatrosses.
Thalassarctine = related to polar bears.
Thalassophyte = a type of seaweed.
Thalassometer = a tide gauge.
Thalassophobia and thalassotherapy (self-evident), and one I like better...
Thalassophilous = fond of the sea.
Addendum: Lockwood adds Panthalassa, the "all ocean" that surrounded the "all land" continent of Pangaea.