What this really kills is the Harbor + CH + city center or other district triangle which was +6/7 gold and made coastal cities attractive as long as there's a river and a dozen or so workable land tiles. Now coastal cities are pretty much directly inferior to inland cities once you have a place to build ships.
For those of you understating the value of a navy, you probably haven't tried it. Sure you can ignore navy and build a huge army, but focusing on ships is very powerful, naval melee units can capture cities, battleships and missile cruisers have 3 range, from mid game on navy steamrolls much faster than land units because your ships all have 5+ moves and no rough terrain and you can bombard heavily fortified cities for a turn without taking any damage, you do need a few land units to take inland cities but once you have wiped out their units and coastal cities its not an issue. That being said you do only need 1-2 cities building naval units and the buildings are really weak, Shipyards in particular, the production scaling with adjacency sounds nice but in practice it is very difficult to even get +3.
What harbors really need to be competitive is some sort of CS boost, just sharing the mercantile one with CH would work, just comparing a +2 harbor to a +3 CH is questionable but when the CH is giving you +11 or +19 gold there's no contest. Some sort of buff to the buildings would be good too, I'd really like to see something like +1 gold from sea resources from a lighthouse, +1 prod with shipyard, maybe +1 food or +2 gold for a seaport.