Water taken directly from the water block should have a chance (~50%) to poison the player.
To purify the water you can:

• use a charcoal filter - https://rlcraft.fandom.com/wiki/Charcoal_Filter
• boil water bottle in the furnace or campfire and receive a new item - "Purified Water Bottle" - it should have 1 thirst point lower than water bottle since you know, water boils, etc.

Another idea is to add a new block - Water Purification Station or Water Filter - crafted with sand, gravel, and some cobblestone - where you can put up to 3 bottles, like in the Brewing Stand, and purify the water, but it should take some time (20 seconds?)

This is how it looks it some other mod

requested and talked about on curseforge a lot and now added a purification system :) 592837b