"you can't own below the low tide line.... " sounds an awful lot like "property owners can own land out to the mean low tide line. " They can own out to it, but not below it.

Since I can be a tiny bit dim sometimes, I looked at the pictures on the helpful link you provided and my conclusion is that you two are essentially saying the same thing.

So say Pam and Dave get their windlass working and anchor out in Maine somewhere. They would be perfectly within the law provided Drift Away is in a spot where she is floating at mean low. Now if she runs aground, all bets are off. But that's not going to happen, right Pam and Dave?