Watertemp. The CPU,GPU temps jumping all around the place according to the load. Approximately 15° in 2 second, idle to full and back.
+1 to this. You can use a virtual sensor to combine CPU and GPU temp into one and use the 'highest temp' function so that if either temp climbs it activates the fan curve but as has been said it just leads to fans ramping up and down for no purpose. Water temps change slowly and the fans being controlled cool the water, if the water is still cool the fans speed up for nothing.
To get an easy water temp you can stick a standard flat sensor to a radiator or water fitting.
Also, a data source can totally be used in more than one controller. I use a single data source (a virtual sensor detecting air / water delta) for 3 different controllers.