It sounds a little overpowered
maybe they should have made it only available to cities on adjacent to both river and coast.
It IS overpowered. For other civs, most of the time only roughly 30% cities have levees, with dykes, in a typical game 80% of cities can benefit because it is so flexible. So it does not just merely add hammers to water tiles, it allows way more cities to benefit. So the benefit brought by the UB is more than double of the one it replaces.
And lack of hammers is almost always the sore spots of coastal cities. Dikes are similar to Maoi Statues, except they are even better because it also gives +1 hammer to a river tile or inland lake tile, even this tile is not located right next to the city, and it is not a national wonder which can only be built in one city. This is just sick. How can a 180-hammer building that can be built almost everywhere be more powerful than a national wonder costing 240 hammers?
I think to be more reasonable, they should first take the inland lakes out of the equation. A city should not be able to build dikes without river or water tiles right next to the city, and an inland lake tile cannot benefit from dikes as well. Also, only the water-tiles of a pure coastal city can benefit, and only the river tiles, not the water tiles, of a non-coastal river-city can benefit from the dikes. So only when a city is located at the river delta (ie. the river tile at the coast) both types of tiles can enjoy the +1 hammer bonus.
Right now when I play Dutch I just beeline steampower, build the dikes, then trigger a golden age and all those dike-cities are like on steroid.
