Best source of Pristine Water is Fighters Guild quest "The Dangerous Past", the water is located in nearly every urn in the tomb, you can collect up to 70 of them here. So many urns also increase the chance to find some useful racial motif.
"The Dangerous Past" is the second fighter's guild quest and it's located in Elden Root in Grahtwood. It can be started right after you rescue Merric from daedra Doshia in Auridon's quest "Anchors from the Harbor".
But to start "The Dangerous Past" you must visit Fighter's Guild in Skywatch.
I mention that because it's hard to find this quest starter when you join Fighter's Guild after you, for example, explore whole Auridon.

Hmm not as many people as I hoped. I'll give another tip.

Insight: Not sure how many know this. If you're using a weapon while attacking with magic, you can speed some spells' casting times by pressing or holding down the button for the spell (1-5) while in the middle of the first attack or right at the end of it. For example, say it takes 3 seconds for the longest heavy attack, you hold your next magic button down at like 2 seconds. This seems to work best with heavy attacks. You can combo spells after light attacks or heavy attacks and the opposite way too.

As far as dps this helps you kill things a bit quicker and start out your initial assault with more power. For my nightblade, I can use the cc spell with like 1/4 it's casting time after an arrow shot. It's good for pulling and ccing one of the other mobs around them. This doesn't seem to effect all spells the same.

I thought about whether or not this was a bug. Considering how some npcs fight? I'm gonna say no. Off the top of my head I know the dwemer spiders can use their thunder after their light and heavy attacks really fast. Hell they use it in between. A group of three of them can be very deadly for this very reason in veteran areas.