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.