Anyone with enough ranks in Engineering can make it.
There are no rules for holy water, only for a holy water grenade. So, yes. They can pray over water to make it holy water, which can then be used to stave off thirst and put out fires. But in a holy way.
I mean, there are rules, they're just rather vague.
A holy water weapon is infused with the blessings of one or more good-aligned deities (most commonly Hylax, Iomedae, or Sarenrae within the Pact Worlds, though devoted followers of any good-aligned deity could create such weapons). It damages only undead (regardless of alignment) and outsiders with the evil subtype, and even those creatures suffer no effect (and show no sign of their nature if it is not already obvious) with a successful saving throw. Crafting a holy water grenade requires the blessing of formally trained priests of a good deity, though a character of any alignment can do the actual crafting.
If it's a home game, I would say it's up to the GM how that actually works mechanically. If it were me, I would definitely let someone with the Priest theme do it, and probably the Divine Champion archetype as well, as long as it's for a good deity. I wouldn't assume that as a player, based on the wording "formally trained priests". But personally since you still have to craft the grenade separately with engineering I don't see a need to make someone jump through too many hoops to get the holy water, as long as they aren't trying to abuse it somehow.