Usually tha waterblock surface is 1°C or 2°C above *radiator* ambient. If not, water doesnt remove heat efficiently enough. (just checked my Maze2 surface - its ok
)
And usually the *inside* of the case (when closed) is about 4-5°C above *radiator* abient. But putting a HSF on your waterblock you're just adding heat to it.
That, unless you've got a 'tornado' kind of case with zounds of fans... and what's the point of watercooling your comp if you're about to put extra fans just for it...?
The N-O relay is quite a smart solution IMHO.