[shrug] It's not a very sophisticated strategy. I'd be disappointed if it couldn't find some way to counter it on its home ground. But we've already covered what happens to a dragon that can't play offense effectively--unless you can interdict all intruders with perfection, a purely defensive game will always lose strategically. You'll starve to death if nothing else.
Edit: BTW, if he's not fully submerged, what's to prevent attackers from shooting him in his sleep? The dragon's passive perception is only 23, everyone in the party can beat that except maybe the paladin (if he stays in full plate).
Not knowing when he will be sleeping.
Lair guardians. I have yet to fight a dragon that didn't have them in abundance usually causing a fight at some juncture alerting the dragon to the presence of intruders.
A very complex and difficult to traverse lair involving lots of difficult terrain and problematic entry points like underwater or far up a cliff.
Experiences do differ. I know I haven't found many dragon lairs easy to enter. Usually they are well stocked with guardians in hard to reach places protected by natural hazards.
The white dragon we fought at 8th level was on an iceberg we had to sail to. He had a tribe of humans worshipping him. He had ice trolls in his lair. He had a multilevel lair with that required some climbing. He left underwater through a cave under the iceberg. The cave where his hoard was had most of the treasure frozen into the ground requiring ice mining to free up. The entire area was difficult terrain slowing movement to a crawl. Very nasty lair. That is what I'm accustomed to. Entering a dragon's lair is usually a dungeon area unto itself.
There's no way to determine whether the strategy you offer would be effective unless everything was designed and we played it out. Since I'm not planning to do that, I'll wait for Dave to let me know how his experiments went. I do like the owl idea. I might try to use some in the future. Seems like an under CR creature to me, but I'll always take advantage of such things while I can.