The cubes from the "Show invisible blocks" option of the structure block have a kind of underwater effect when there is water behind them.
How to reproduce:
1. Fill the ground with water
2. Place a structure block in save mode over the water
3. Set the "Show invisible blocks" option to "ON".
4. Look at the water with the cubes in between
You can also check the difference by looking at a block with the cube in between. If you place a structure void on these cubes, the red cube will also have a water effect.
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Thank you for your report!
We're actually already tracking this issue in MC-167342, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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I donΒ΄t think its a duplicate because he doesn't mention the cubes for the invisible blocks just the structure outlines
I cannot reproduce.
[media]

You can. I have marked the relevant cubes for you.
[media]I mean the part that can be viewed through the block is valid.
[media]
I have attached the issue you mentioned to MC-167342.

Ok thanks