In a recent game (#78017331 on oregon), my opponent bluegrins played a Seahag but it didn't cause me discard or to gain a curse. They were able to continue their turn as usual though.
The one turn they played Sea Hag (their turn 13), you had a Lighthouse in play, which reads (among other things):
While you have this in play, when another player plays an Attack card, it doesn't affect you.
So everyhting is as it should be, the Sea Hag is not affecting you, so you don't discard from your deck and don't get a Curse on top of it.