One little tidbit when you water cool. Not only does the CPU fan actively move air across the, but also the power supply MOSFETs on the mainboard that supply stable current to the CPU. These MOSFETs are arranged around the CPU and are designed to work at higher temperatures, but still are reliant on the CPU fan moving air across them. So make sure you have some sort of active cooling still happening in your case, albeit a large, slow-spinning case fan works better for this.
Also, you can always just disable the sensor in BIOS if you know you do not really need it. Some water pumps also have a connector that allows the pump to report its RPM to the CPU sensor so the mainboard can monitor the pump activity (serves about the same purpose allowing the computer to warn you when the pump is not running).