Evolving weapons happen by combining a max level weapon with any level of a specific secondary support item. Once you have both of those items you must then kill an Elite enemy and open the chest that it drops. The combinations for the Santa Water is the following:
La Borra – Santa Water+ Attractorb
Santa Water is unlocked by default and Attractorb is unlocked upon finding a Vacuum dropped by a light source for the first time (Blue orb that when collected brings you all gems on the stage).
If you're interested in seeing what the combo is for the other weapons they are listed below in the spoiler tag :)
*** Spoiler - click to reveal ***
Hellfire – Fire Wand + Spinach
Death Spiral – Axe + Candelabrador
Bloody Tear – Whip + Hollow Heart
Thousand Edge – Knife + Bracer
Heaven Sword – Cross + Clover
Unholy Vespers – King BIble + Spellbinder
Soul Eater – Garlic + Pummarola
NO FUTURE– Runetracer + Armor
Thunder Loop – Lightning Ring + Duplicator
Gorgeous Moon – Pentagram + Crown
Vicious Hunger – Gatti Amari + Stone Mask
Mannajja – Song of Mana + Skull O'Maniac
Valkyrie Turner – Shadow Pinion + Wings