@Paradoxxx: adjusting the fans to the temperature of the CPU/GPU will not make much sense. For example if you start a large program it will give a full load for a few seconds and the temperature will rise up - but also speeding up the fans in this scenario is worthless because the water temperature will not have changed at all in this short period.
It may not be ideal but I think it can work, the fan speed can be set to "incrementally" increase with time right? During the start of the large program the fan speed will increase just a little and then return to normal. Or in the case of gaming, my GPU temps are usually at 37C idle, upon load it rises +10C as such I can increase fan speeds accordingly. If there was a flow/temp sensor all in one I would consider purchasing one.