See Transparent Blocks Through End Portal Off A Chunk Border

If water is on any Y level that is not the top of the chunk, it will be visible through an activated end portal, it will also flash rapidly.

 

Steps to replicate

1. Put water on a Y level that is NOT the top of a chuck (Y level divisible by 16 minus 1)
2. Build a portal so the water can be seen through the frame
3. Activate the portal