When the player has no effect, they are able to see the outline of shapes in the water fog / murkiness.
However, if the player is given blindness while using commands and underwater, the player is able to see much easier and clearer than without the effect.
If the player is given the blindness outside of the water or swam out of the water and then swam back, the player will be blind at first, but then gradually will be able to see better
This appears to be a duplicate of MC-167763, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.