What would the future look like if Pathfinder magic and monsters suddenly appeared on modern-day Earth?
Spoilered for length, and maybe content...
So I'm watching South Park on my laptop, and it's the episode involving legalized medical marijuana in Colorado. One of character, in order to partake, gives himself testicular cancer by putting them in the microwave. This results in his balls swelling up so big he can ride them "like a hippity-hop." His friends see him lighting up, and ask him to share. He refuses, but explains what he had done to be able to buy the stuff.
It's at about this point in the story that my daughter, about 3 at the time, crawls up in Daddy's lap.
She looks at the screen and sees several men bouncing down the street on their "hippity-hops."
And she exclaims, "It's a sack race!"
The "Swarm" subtype is immune to weapon damage (at least for Fine or Diminutive creatures), and it seems this is due to the fluid nature of the swarm. So I think objects in liquid and gas form are also immune to weapon damage.
To answer the original question, look at it mathematically. HP=(H)*(P), and by the commutative property, (P)*(H)=ph. Water has a ph of 7, so it should have an hp of 7 as well.