In the immunity rules, we read:
Immunity to critical hits works a little differently. When a creature immune to critical hits is critically hit by a Strike or other attack that deals damage, it takes normal damage instead of double damage. This does not make it immune to any other critical success effects of other actions that have the attack trait (such as Grapple and Shove).
My interpretation of this rule is that the normal doubling of damage does not occur, but any other listed affects of a critical success do occur. I believe this is the accepted interpretation of the rule, as well.
Now, about the spell Hydraulic Push. This spell reads a bit unusual, because it normally does 3d6 damage (and 5 feet of knockback), but has a listed critical effect that instead does 6d6 damage (and 10 feet of knockback). This is notably different from the standard/basic critical effect of rolling normal damage and then doubling it.
When Hydraulic Push crits against a crit-immune creature, how much damage does it do? And more generally, are extra damage dice listed under a critical effect applied to crit-immune targets?