Knowing the way our local government works, I’ll put money on the fact that they’re counting your waterway as part of your land.
That was more or less hypothetical as the only flowing water on my property is when the snow melts. I do have an easements along the frontage for a county highway and another along the back of my lot for a private road that is in fact the only access for three more houses behind my lot. I did some digging and I pay property tax on both easements. The county road bites as it is over 300 feet long and 24 feet wide! The private road is half that in area and the three home owners are responsible for upkeep so it's not such a bad deal as it also gives me access to the back yard for trucks and such.
Regards,
GtG
PS Even with all the nibbling along the edges my lot is about 2 acres.