Summary:

When you position your camera inside of a boat at the correct height, the water will be visible inside the boat. The inside of boat blocks the rendering of water to prevent it from appearing to be flooded. (Also happens with other transparent blocks MC-118561) however this doesn’t seem to work if the camera is positioned inside of the boat. This also causes MC-224703 and MC-224242 to not happen.
Relates to MC-165181 and MC-47636.

Steps to reproduce:

1. Place a boat inside water
2. Go into spectator mode
   I recomment to turn the speed down using the mouse wheel
3. Fly slowly down into the boat

Steps to reproduce 2:

1. Place a boat inside water
2. Set third person view
3. Place the camera in the boat at water level

Image: