Interesting thing about D&D holy water: if you consider it carefully, doesn't quite add up.
Bless Water says that it imbues a one-pint flask of water to create a holy water flask. A holy water flask weighs 1lb in the equipment list. But a US liquid pint of water weighs 472 g at 62 °F, which is slightly more than a pound, and that is not factoring in the weight of a flask. Since the flask breaks on impact, it is probably glass. Glass is pretty dense, its weight is not negligible. A one-pint glass bottle weighs over half a pound: a one-pint glass flask of water, logically speaking, should weigh one-and-a-half pounds minimum, but a holy water flask weighs one pound flat. It isn't a matter of the PHB not bothering with tracking weights that small, either: a one-ounce vial is listed as weighing ⅒lb. So the actual volume of water blessed by Bless Water must be somewhat less than a pint to allow the full flask to weigh a pound rather than a pound and a half. If I were to venture a guess, a "holy pint" is ten fluid ounces, making it slightly smaller than a bottle of Coke.
The price of the bottle used is also in question. Glass is not cheap. A one-ounce vial costs 1gp, and a flask of holy water is more than an ounce of liquid, so no doubt the bottle used for holy water is worth more. But it is never factored into the price: Bless Water's material component of five pounds of silver costs 25gp, and Good-aligned temples sell the holy water "at cost" for... 25gp. The authors never listed the price of the flask, they clearly didn't intend for players to think about it, despite glass clearly being valuable in this world. The reasonable conclusion is that Bless Water consumes somewhat less than five pounds of silver, and some of that silver is simply assumed to be spent already on a bottle.
Which brings us to this feat. Holy Water in the PhB is clearly not meant to be thought of in terms of a specific volume of liquid, and the temporary holy water created by this feat even less so. The cleric releases a burst of positive energy, which then channels through nearby water. This deals 2d4 damage right away, and another 2d4 damage for 1+ChaMod rounds. It does not deal more than 2d4 damage if there is a lot of water around the target: if that is what the feat were intended to do, then there would be at least some way of calculating how much damage that would be. We don't even know the real volume of a "holy pint" of water, as it clearly isn't a regular pint of water. But it also still deals damage in subsequent rounds in a way that regular holy water wouldn't.
Since it doesn't specify the amount of water necessary to do the damage, I would just treat it as 2d4 damage initial, plus another 2d4 damage per round for the duration. At the DM's discretion, the subject might avoid the damage if it is really dry.