If there's a river, a large section of it must be surrounded with a wall, too. New Orleans did exactly that. Many other coastal cities already have levees aroun

If there's a river, a large section of it must be surrounded with a wall, too. New Orleans did exactly that. Many other coastal cities already have levees around their rivers to prevent seasonal flooding, and there's even a whole country in Europe that is famous for being below sea level, so there's nothing new here. The walls are not even noticeable when there are even taller buildings nearby. Smaller streams, leakage, and sewage can be pumped out mechanically. The harbor will also have to remain outside of the wall. In the event of a tsunami, any boat left in the harbor will be toast. It's a cost that the city is willing to bear in order to protect the majority of its residents from an existential threat. If what you're worried about is sea level rise, not a tsunami, then the harbor is not a problem at all. You just build the harbor higher so it stays above the new sea level. The massive concrete backside of the harbor could even form a section of the wall.