It seems that the top of water doesn't get rendered if there is transparent block (this also includes slabs for example) in a 1 block radius one block higher than the water. This makes water look pretty strange if it is tilted.
- Fill a 3 x 4 area of a solid block like stone
- Place water or lava below middle block at the edge of a 3 blocks side
- Move inside the lava or water block and rotate in the direction of the area you filled
Use the following commands without moving
- Optional to make sure the water or lava block does not move you