Hiya,
Some seem easy to slide i guess
To put your mind at rest though, just power up the waterside of things
If needbe (in case of no 2nd psu) remove the mobo power strip, with psu Off, bridge the lone green wire to a black one (you can then use this psu solely to run whatever you like (bar the mobo/gubbins etc)
And place some tissue etc around the bridge/cards, just in case, but obviously leave enough on view to watch for a leak
ps : I've had water allover a mobo years ago (think it was an asus A7v lol) still worked til the day i threw it out (i do not recommend this though obviously) hehe
Any doubts about the atx psu bridging if you want to test the loop, either post back or google, it's easy but just make sure you're 100% before attempting anything like this
pps : If you do get a leak (imho very unlikely, but you never know) use tissue etc to remove what you can see, then with all water loop/power off etc, use a hairdrier on low setting to remove any you cant see, on vidcards you can apply a decent amount of heat (you ever felt these critters during gaming ? toasty pcb lol)