And I think that's the answer he will get. If he gets one at all.
OP, there has been multiple threads and comments about this very subject on duck boards online. The two most often asked are the one you asked and the "is this channel/creek/slough/bayou public?"
Common answer to yours is the high water mark is still the property line of an adjoining landowner, whether it is dry or 15' under water.
The answer to the other one is, if it's navigable (and listed/stated as such by COE) then it's public. If it's non navigable then it's private water.