I used to add a bit of my breastmilk if it didn't puree well but actually yeah the water you boiled/steamed with actually works just as well so I do that now
I add a little splash of boiled water depending on whether it needs it. If you add formula you can’t freeze it.
Some purées don’t really need anything added to it to be honest, I find with most soft vegetables they’ll just puree on their own. But if you want to, you could use water. Formula would be a good idea to get some of their milk in but not if you want to freeze the leftovers.
A bit of the water you boiled them in is fine, it had a lot of nutrients in it. Also you don't need to blend all the way till it's smooth if your baby is 6months