Is it still "landing" in a seaplane? This probably sounds like a silly question, but I was watching floatplanes land on Lake Union, Seattle, and it got me wondering whether a plane that touches down onto water is still talked about as "landing" in aviation parlance. 

Is it?
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.