pathfinder 1e – How does underwater combat work with Soul Knives?

The normal rules for water combat would apply, however you can have the weapon deal piercing damage which will allow you to do normal damage if you have a swim speed, firm footing or freedom of movement as per the water combat rules.

When shaping her weapon and assigning abilities to it, the soulknife chooses whether it will deal bludgeoning, piercing, or slashing damage.

Mind Bolts from the soulbolt archetype should follow the range weapon water combat rules.