The story
- Proto-Indo-European*wed-
- Proto-Indo-European*wódr̥
- Proto-Germanic*watōr
- Proto-West Germanic*watar
- Old Englishwæter
- Middle Englishwater
- Englishwater
- Englishmelon
- English todaywatermelon
Endocentric compound of water + melon. Compare French melon d'eau, German Wassermelone, and Spanish melón de agua. The meanings referring to environmentalists and behind-schedule projects derive from the idea of having a green (eco-friendly, on-schedule) outward appearance while being red (socialist, behind schedule) in a hidden way comparable to that of the fruit.
Etymology from Wiktionary, CC BY-SA 4.0.