An adjective to describe "being at sea"
You are looking for afloat:
- floating
- out at sea
“Afloat,” though, all too conspicuously lacks any connotation of purposeful motion. The nautical term you seek is, I believe, “underway”:
a. Naut. Of a vessel: under way; having begun to move through the water. [OED]
What comes to mind is afloat.
In Plato's ship of state metaphor, a ship is afloat, helmed by a captain ...
afloat: floating on water; being at sea