You're misunderstanding me.
The point is that while some planes - and birds - can go over some mountains not every plane - or bird - can go over oceans. The ability to cross oceans does not inevitably follow from the ability to go over mountains. (See also: Legs.)
Also, there's no real reason why dragons can't go over the ocean...I urge you and others to either look for more info or show me info that proves Dragons can't do anything..
We are not going to give Dragons all powers not explicitly ruled out by the "lore."
And there my point is not that Dragons are definetly incapable of flying X distance. (I said "
if we want...") Just that there's an easy "realistic" - note the quotes - rationalization available for explaining why they can't.