If you do any waterblock on whatever part...
Always warranty is VOID. but If you keep the stock cooler, all mounting stuff. In case some defect (THAT IS NOT WATERCOOLING RELATED, like leaks and further below mentioned damage) arises. Other damage that are user inducesld stuff scratched , over tightened srcews, cracked pcb, water stains or corrosion.
So if other defects may arise: Remove waterblock, really good and thoroughly clean everything, put back the stock cooler (tim applied, orininal thermal pads). Send it in for RMA, if no clear damage (as mentioned) you have good chance that RMA is no problem.