Grass blocks spread their grass to dirt blocks underwater if the light level is high enough. Can be tested by setting the gamerule randomTickSpeed to 1000 and by placing a grass block amongst several dirt blocks underwater. Especially noticable in swamp biomes, which can be completely covered in underwater grass after some time.
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duplicates 1
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This behaviour can be caused by Optifine, so I think MC-25642's reporter simply had that installed.
The water translucency has been increased in the latest snapshot, and this apparently is a result of that. Still doesn't mean that it's intended.
There have been several similar bugs reported for MC 1.6 (>>> MC-25642), which were closed as dublicates and not reproducible. I'm not sure if a separate ticket for the issue seen in the 1.13 snapshots makes sense, I made one just in case.