[So Merlin's had zero luck with being able to sleep lately, and Arthur disapproves. The entire island being flooded and watery just gives Merlin the inability to fall asleep consistently and stay asleep.]
[And Arthur disapproves forever. So if you happen upon the treehouse they're staying at, one of the nights, you'll find Merlin sleeping restlessly, and Arthur sitting at the entrance, sword at ready, and just glaring at anyone who so much as speaks above a whisper or dares to splash the water. Don't you dare wake him up, is all he's saying.]
[Although it's obvious beneath that protectiveness that a lot of it is fueled by not only affection for Merlin, but also a sense of guilt and uselessness. He can't protect him from crappy sleep, after all. Mrr.]