Do not depend upon tax and GIS records; they can be wrong. Yes, they are often the best maps we have to work with, so we use them anyhow, but they do not describe legal boundaries. A survey does. You may not need this land surveyed, but a surveyor who has worked nearby may be the best person to easily shed light upon your question. If they know the answer without having to research it, they may not even charge for their advice.
You can see the confusion that's common with what the attorney said - they are likely just reading the statute as it's written - clearly ambiguous. I would speak with a surveyor that's familiar with the area and has shot property with bodies of water. Guys in the field are typically more experienced in the day to day than guys driving desks....
But - expect variables anytime you bring water into an equation.