If it was me, I would cube my water. You can bring it to a boil, then 'hot cube it' in a HDPE cube like you would for hot wort, and then store it and when needed, it's ready to go. Just pour it in. This has the advantage of being at room temp, or you could refrigerate if you have the space to cool it down, but with your fermenters you could easily pour the hot part straight into fermenter at boiling hot, then add your room temp water from the cube.
Set fermenter to temp you want, then wait till it reaches there to pitch your yeast.
Saves buying water, and if you fill a HDPE cube up and close it, it will be fairly sanitary if not almost sterile.
edit: On top of that, it's much easier than boiling and cooling down. You also don't need to fill the cube to the brim, as you're keeping straight water oxidation isn't an issue. So say you know you need X litres of prepared water, you could measure it out, bring it to a boil, fill the cube, close it and leave it to cool overnight and then it's storable for a fair while I would assume.