it looks like it isn't possible to attack a castle from the shore using a flyer
I tried from several different locations and was only able to attack from the land.
http://twitpic.com/1xzgq2/full
Flyers can go between land and water anywhere unless they are stacked with a non-flyer. If they have a non-flyer they must go via a dock. is that expected behavior? It seems to me that a flyer should be able to cary one non-flyer unit between water and land without using a dock
I noticed one exception, flyers can carry a hero between water and land at any point.
I guess this is the same rule that governs flyers movement over mountains. I created a bug for that too