There could be a band where lots of powerful storms are generated. That should take care of any airships. Storms at sea can spring up within minutes and be on you faster than you'd ever beleive. Some you can outrun, and if that direction is always towards the known land, eventually airsailors will stick within sight of land.
Always remember: just because someone can do something rarely means they will do it. We had ships capable of oceanic travel long before we thought we did, but people seldom bothered. It was expensive both in lives and materials. If the last two ships that tried to reach across The Great Dark Sea were lost with all hands, it's going to be damn hard to find crew for a third. There might well be mutiny if the captain decides to change plans mid-course as well.
There might well be phenomenon that the sailors know nothing about. Maybe there is a type of flying swordfish that, when it sees the airship drop low towards the water as it's very likely to do, thinks it's prey and attacks it. BOOM goes airship. Yum go the sharks.
We just found out that the 100' ship-destoying waves that sailors claimed existed not only really do exist, but are a heck of a lot more common than we ever thought. We've had space photography and all this other tech and we only just now found this out. Surely there are other natural phenomenon in a magical world that are just as devestating.
If you want them to find out about the lost continent, try this: after a great week-long storm, several clumps of junk are washed up on shore. The local fishermen just ignore it and toss it away but your scholar might notice that the plant samples are like nothing else know, period. There might be a corpse of some strange, but natural, animal.
He could come across a rutter (pilot's journal) from a long-dead pilot who claimed to discover this 'unknown green land' across the Great Dark Sea but was dismissed as a madman when he could not get back there. What he ddidn't know is that he managed to get caught in a current that only occurs under specific conditions that swept him to the other land. Based on the rutter and the information on the storm, the scholar manages to put two and two together and discover the narrow time frame they have.