If you have airships unlocked, you have completed "The Ul'dahn Envoy" as you say.
Next would be "Call of the Sea (Ul'dah)" issued by the Serpent Officer in Old Gridonia (8, 11).
This will send you to Limsa Lominsa Upper Decks (When leaving the airships, go to The Drowning Wench) and talk to Baderon at (7, 5).
This will start you on the quest "It's Probably Pirates" to queue to Sastasha.