JRC Global Surface Water

The EC Joint Research Centre's Global Surface Water dataset classifies every 30m Landsat pixel on Earth's surface, monthly, back to 1984, into water/non-water — then rolls that up into occurrence, change, seasonality and recurrence layers spanning four decades. It is one of the most-cited remote-sensing water products in existence, and freely browsable and downloadable with no registration.

For anything asking "how has this reservoir, lake or floodplain's water extent changed" — dam-safety monitoring, reservoir sedimentation screening, wetland assessment — this is the dataset to start with, and it's built directly into the Global Surface Water Explorer and Google Earth Engine.