Wiktionary
underseas
adv. Under the surface of the sea or ocean, underwater.
Usage examples of "underseas".
Members, Sellars, Underseas, and all the people of all the groups anywhere.
There are three distinct but inevitably interlinked elements in this story: the Merchant Navy (officially the Mercantile Marine) and the men who served in it: Liberty Ships: and the units of the German forces, underseas, on the seas and in the air, whose sole mission was to seek out and destroy the vessels and crews of the Merchant Navy.