Placing a Light Block above the water prevents the player from being able to climb out of it.
I've tested this on Switch, Windows and Android, on both 1.17.30 and 1.17.40.21

Steps to reproduce [by EVGENSYPERPRO]:

1. Place the water block at the water outlet level.
2. Place a light block above the water.
3. Try to get out of the water onto a block adjacent to the water.

Observed results:
The light block prevents the player from getting out of the water onto the block.

Expected results:
The light block will not affect the player in any way.