What are the requirements for national parks?

The problem is for sure that your diamond shape is diagonal instead of vertical. (Note that the Spring 2017 update, or an update prior to that, fixed the Civilopedia entry to specify that the diamond must be vertical.)

For this to be a legal configuration for a national park, it would have to include your Niter mine, the two land squares you want, and the rightmost of the water tiles you want to use. However, this is also conditional on your tiles being appealing enough since they are not natural wonders or mountains:

You're probably SOL unless you want to scrap your Niter mine and possibly plant a forest instead (I'm not sure if new-growth forest can be a national park or it's only old-growth (never cleared or planted)).

Frankly I think national parks are either underpowered or too hard to establish.