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.
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.
Not in French it isn't. French for landing is 'aterrir' which roughly translates to 'return to the land'. Using this verb for a sea 'landing' would sound silly to them so they use 'amerrir' which means 'return to the sea' (Mer being French for sea, terre is French for ground). A landing in the Moon would be 'alunir'.
This interesting fact was mentioned by the instructor on my French audio course.
No. ICAO defines a landing like this:
From the beginning of the landing flare until aircraft exits the landing runway, comes to a stop on the runway, or when power is applied for takeoff in the case of a touch-and-go landing.
And a runway like this:
A defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft
Wikipedia suggests that "landing" on water is referred to as "alighting"