I know nothing about watercooling pumps but i hopefully know enough about electricity to answer.
I you are just looking for a replacement you can find a Swiftech relay kit at the following URL
http://www.frozencpu.com/products/2124/ ... it_II.html
(2nd URL after googleing "swiftech relay" )
If you want to make one yourself, any kind of 12v / 120v relay would be good. If you want an example see (Montreal based electronic surplus store but you should be able to find that in any electronics store) :
https://www.addison-electronique.com/ca ... s_id=90285
which i strongly advise to couple with a socket to simplify wiring
https://www.addison-electronique.com/ca ... s_id=90324
Add some kind of diode to be sure no power is ever fed back to your power supply, wire everything using a molex in a nice utility box, play safe when playing with main and you are good to go.
I did a similar setup in a computer to control a 180mm/120V fan and it works perfectly. Then again, be sure to know what you are doing.