See screenshots, they're self-explaining. This rendering error happens when looking at water through ice, portal, stained glass, etc. ..., only saw this on the horizon of oceans and only happens in certain view angles. Most likely strongly related to MC-34795, the only difference I see are the big, odd bright rectangles replacing parts of the water when looking at oceans.

EDIT:
Happens when looking through portals, ice, etc. too

EDIT2:
As of 13w41b, the rendering errors appear black.

EDIT3:
As of 13w42a, the black errors no longer appear, but errors can be seen at the water surface: In certain angels, the surface texture turns completely invisible, see "13w42a.jpg"