Probably one of the most well known effects on the console. Some zones in Sonic games have translucent water that covers the whole width of the screen:
The effect is achieved using a raster effect that rewrites all palettes at the water height:
You may notice there are a few garbage dots in the second screenshot: these are the so-called "CRAM dots", which show up wherever a color is written to the palette (and are visible because this is being done in the middle of the screen). The game covers this up using the ripple sprites, which are noisy enough that the CRAM dots get lost within them.