Rijkswaterstaat WaterWebservices (Netherlands)

What it is: The Dutch water-data service, with a multi-decade archive of 10-minute water levels.
Publisher: Rijkswaterstaat.
What OTC takes: Gauge records that OTC fits. No harmonic constants are published. OTC fits constants from the long observed records. Rijkswaterstaat also publishes an astronomical (predicted) water level, which OTC can use to cross-check its fit.
Licence: CC0 (Creative Commons Zero), as stated on the Rijkswaterstaat water-data page.
Attribution line OTC uses: “Contains data from Rijkswaterstaat (CC0).” The licence does not require it; OTC credits the source anyway.
Access and watching: Keyless JSON web services (DDAPI 2.0), requested in pages by period. A weekly fetch of new records.
Coverage: The Dutch coast, the Wadden Sea and the estuaries. Full-year records are available for backfill.
Rank: Gauge fit.