When an air block is under a water block, the bottom faced texture of the water is transparent until passing into the air gap. When a non-waterlogged block is submerged in water, even if it has no bounding box (ladders, signs, levers, flowers, other plants, buttons, pressure plates, or more related), the water texture is transparent on all sides until passing into the space, or looking at it from an angle where the player can see air and water in a straight line.

Video Example:

https://www.dropbox.com/s/9r6247r7qdrdjbm/Clear%20Air.mp4?dl=0