Water cooling only really pulls away in terms of performance at the higher end of temperature loads so I wouldn't worry too much about idle temperatures. My full loop is performing on par with the H100 I was using previously when at idle, give or take a couple of degrees. But.....My new loop mops the floor with the H100 when working heavy temp loads produced when over clocking.
Reseating the CPU block sounds like a good place to start, just in case that's the issue.