Are you sure Pan Am and Imperial/BOAC used stewardesses on the boats? I've got a suspicion the crews were all-male but could be wrong.
I've seen at least one photo of BOAC stewardesses on a flying boat, and I know that PanAm used them briefly on the Boeing 314s between SFO and Hawaii. "Briefly" largely because the Boeings were quickly replaced by DC-4s and then -6s.
But you're right in that PanAm didn't hire any stews until 1944--they were late into that game--so most of the serious flying-boat use before that was male-staffed.