Actually that was my thought. Flooding the whole area makes for a great defensive measure but how long would it take to reverse it after military action is over? Is it even possible? Would a permanent moat not serve the same purpose? Even if the area was flooded, couldn't the enemy just go for the same "starve you out" tactics anyway?
The whole defensive measure seems more like a one-time trap for a powerful invader. I can only see it working once, used to destroy (at least partially) an attacking force.
The map itself is quite fine! Nice clear and vibrant colours. Although the grid approach to the building placement I am not a fan of, it does illustrate the built up areas so does serve that purpose!