The name Ansium gives me a Latin (Roman) vibe.

Ansic Ocean?
Ansine Ocean?
Mare Ansii?

From what little I have read on the CWBP, Ansium is the entire region surrounding the sea? In that case, maybe a contraction of intra (inside) and ansium (or ansic, or ansine) to make it Mare Intransium, Intransic Ocean, or Intransine Ocean?