Is it still "landing" in a seaplane?
Yes. One 'official' example is from the FAA's test standards for land and seaplanes, which includes these tasks for seaplanes:


  Task H: Glassy Water Approach and Landing
  [...]
  Task J: Rough Water
  Approach and Landing


The FAA's Seaplane, Skiplane, and Float/Ski Equipped Helicopter Operations Handbook also uses it; Chapter 6 is called "Seaplane Operations - Landings".

Interestingly, the word "landing" itself isn't defined in the FAA's glossary or in 14 CFR 1.1 (the US air regulations general definitions). Presumably it's considered to be too 'obvious' to require a definition.