The bug

The underwater overlay texture, used for covering the screen when underwater (located at misc/underwater.png - not to be confused block/water_overlay.png, which is used for the sides of water against transparent blocks) is likely currently the most outdated texture in the entire game, since said texture was introduced to the game in 0.0.12a from May 2009 (tying it with bedrock and therefore MC-160720 given their coinciding implementation - and considering the old water texture was removed in 13w02a and was relegated to being a placeholder in 0.0.19a water is the outright winner). Given that water has been updated very many times since May of 2009, the underwater texture is nowhere near reflective of the current water texture, and is direly overdue for an update (arguably even in Programmer Art).

The fact that this has been overlooked for so long is likely a product of MC-128337, making this texture prohibitively difficult to see.

How to reproduce

1. Create a fully underwater region within a cuboid of white concrete (other textures are too noisy to allow the overlay to be clearly seen)
2. Note the appearance of the water overlay texture, and take close note of it and its geometry
3. Load up an early Classic version with water (0.0.12a - 0.0.18a_02: the launcher has 0.0.13a-dev and 0.0.13a_03)
4. Note how the water texture in this version has the exact same texture as the water overlay still used in modern versions

Expected results

In the over 12 years water has existed, its underwater overlay texture would have been updated at least once to account for other changes made to water's textures. This could have been in 0.0.19a, where algorithmically-generated water and lava textures were added, 13w02a, where animated textures became defined by image files, 1.13, which revamped underwater content, or the Texture Update, which redid a majority of the textures.

Actual results

The texture is the exact same as that used in Classic, which can even be confirmed by extracting the jars and comparing the two textures directly.

How to fix

The resource pack attached to this issue contains somewhat of a fix for this issue which I made using what I had. underwater.png has been replaced by a texture made from isolating the first frame of water_still.png and using GIMP to color it 3F76E4 (Java's default water tint), as the underwater overlay texture currently supports neither tinting nor animated textures (both of which would be appreciated, in which case water_still.png could simply be duplicated for use as the underwater overlay, and the tint could copy from the biome's water fog color).