Water vision also applies outside water while standing in water

I've noticed that too. Hardly seems intended.

Yes, this is clearly not intended if you're exploring a cave system and can give yourself night vision just by standing in flowing water. Players can carry a bucket of water with them to give themselves night vision wherever they want. Clearly exploitable.

Issue1: time of day being ignored and night being ignored still is bright as day
Issue2: if entity can not see the sky still renders has having night vision
Issue3: the y level is completely ignored you can be at y of 0 and it will display night vision
Issue4: head not even in water only waist displays night vision

The current underwater vision is straight up night vision, making this sort of bug occurring and making every light sourcs underwater completely useless.

Even worse, if you have an actual night vision potion effect active in a dark cave, and you step in water, everything goes dark for a second while the water-vision system kicks in. Which is super annoying since natural water pockets exist in caves.

I think the solution for at least some of these issues is to implement the following checks before triggering the water-vision rendering:

1. Check for water at player's eye level rather than their position.
2. Check that the player doesn't already have a night vision status effect.

Those are the most important and most easily fixed issues. As for figuring out what the brightest light level should be, that's quite a bit more complicated, but perhaps lighting updates could somehow be calculated in a second pass, ignoring water's attenuation, and those values used for the maximum brightness underwater? I know that sounds like it'd be super inefficient, but it's just an idea off the top of my head.

This feature seems overpowered. I agree that if this feature is to continue, the player's head (camera) should be underwater. Currently, exploring deep caverns with a bucket of water is a LOT easier with this feature.

seems to be fixed in 18w10a.