Here's a feat to make Holy Water, let me know if you like it.

Consecrator

General Feat (Prerequisites: Level 4+; Cleric or Paladin)

You gain the following benefits.

Ability Score Increase. Increase your Wisdom or Charisma by 1, to a maximum of 20.

Divine Guidance. When you throw a flask of Holy Water, the target has Disadvantage on its Dexterity saving throw to avoid being hit by it.

Consecrate Water. With 1 hour of work using a Holy Symbol and expending 15 GP worth of materials, you can create a number of flasks of Holy Water equal to your Proficiency Bonus.  You must also provide 1 pint of water for each flask created. The holy water in these flasks loses its magical properties after 24 hours.

Revision:

Consecrator

General Feat (Prerequisites: Level 4+; Cleric or Paladin)

You gain the following benefits.

Ability Score Increase. Increase your Wisdom or Charisma by 1, to a maximum of 20.

Divine Guidance. When you throw a flask of Holy Water, the target has Disadvantage on its Dexterity saving throw to avoid being hit by it.

Consecrate Water. With 1 hour of work using a Holy Symbol and expending 25 GP worth of materials, you can create a number of flasks of Holy Water equal to your Proficiency Bonus. You must also provide 1 pint of water for each flask created.

This idea is interesting but really niche. I'd suggest an add-on effect so this feat is not only good against 2 creature types, such as making creatures hit by holy water vulnerable to cleric and paladin spells or radiant damage. Alternatively, let them use their spellcasting dc for the holy water save.

The only benefit this feat has over Ceremony, which cleric and paladin already have, is that they don't need to prepare the spell and an increase in their spellcasting stat and a small advantage when the water is thrown. However, considering how often a character actually needs to create holy water, they probably won't have Ceremony prepared most of the time anyway. With the amount of options available for feats, I can't really think of a reason why I'd pick this one, unless the campaign is very heavily holy water themed and it's a major factor in the story.

As Whatstray mentioned, it could use some additional benefits than just creating holy water. For example, having it available for more classes to have them have a way to create holy water. 

Good points, I revised it again:

Consecrator

General Feat (Prerequisites: Level 4+; Cleric or Paladin)

You gain the following benefits.

Ability Score Increase. Increase your Wisdom or Charisma by 1, to a maximum of 20.

Divine Guidance. When you throw a flask of Holy Water, you can choose between the normal DC and your Spellcasting DC for the Dexterity saving throw. You also add the modifier of the ability you increased with this feat when you deal damage with Holy Water. If the target succeeds on its saving throw, it takes half damage.

Consecrate Water. With 1 hour of work using a Holy Symbol and expending 25 GP worth of materials, you can create a number of flasks of Holy Water equal to your Proficiency Bonus. You must also provide 1 pint of water for each flask created.

Edit: I'm a Ravenloft DM, btw.

You could make it available to all classes by making the prerequisite magic initiate (cleric)

edit: like a feat chain

Yeah, I've been thinking about it, but haven't decided yet. The issue is being able to use a Holy Symbol as a Spellcasting focus, which only the Cleric or Paladin can do. I could just add that as the prerequisite, but it's not as clean.

Quote from alvarojosemarquez >>

Yeah, I've been thinking about it, but haven't decided yet. The issue is being able to use a Holy Symbol as a Spellcasting focus, which only the Cleric or Paladin can do. I could just add that as the prerequisite, but it's not as clean.

This sort of prerequisite is used all over the Bastion rules, so there's already a precedent for it in official stuff. Doing it that way sort of future-proofs it for possible new classes in the future, or subclasses that grant divine spellcasting to non-caster classes (along the lines of Arcane Trickster and Eldritch Knight).

I think an entire feat just for Holy Water is still a bad trade. I would simply let my players use their Spell Save DC for Holy Water. If a PC has the Tavern Brawler feat or is otherwise proficient with improvised weapons, I’d let them make an attack with a vial (1 or 1d4 piercing damage + Dex or Str modifier) and then trigger the Holy Water effect. Or let them hurl it with a sling.
BTW, I made a feat chain about the same topic because I’m disappointed with how 5.5 handles alchemical weapons. Compare this: (inspired by the Pathfinder Alchemist)
https://www.dndbeyond.com/feats/2323986-alchemist
https://www.dndbeyond.com/feats/2323945-chemical-warrior