What sea did the Argonauts cross?

Where did the Argonauts sail to acquire the Golden Fleece?

Colchis

Locating the Golden Fleece
Argo would have to carry Jason and the entire group of Argonauts to Colchis, which was at the eastern end of the Black Sea, south of the Caucasus, in the western part of modern Georgia (the independent state in Transcaucasia, Russia, not the state in the United States of America).

What ship did Jason sail on?

ship Argo

… famous voyage of the ship Argo in which Jason and other heroes sailed to Colchis to bring the Golden Fleece back to Thessaly.