The new Rise and Fall expansion for Civilization 6 adds a new district, the Water Park. It provides amenities like the Entertainment district but can only be built on coastal tiles. Given that a city cannot build both, why would I choose a Water Park which costs more to build and comes late in the game?
You will not unlock the Water Park until later in the game, consequently the Water Park cost more production to build. However all things aside the major difference is a fully upgraded Water Park will provide the same amount of amenities except instead it will be 9 tile radius instead of 6 however the Water Park has to be built in the water on a coastal tile. As an added bonus you get a slight tourism and science boost with the Water Park as well.
You are better off with the Water Park but only for cities with coastal tiles and assuming you haven't built an Entertainment District earlier in the game!