if the fan doesnt turn on then its not the water pump, it looks almost identical to the prob i solved by swapping the 12V and GND terminals, AC light on but not DC, does it power the beagle if connected? The DC light doesnt need to go green for that)

also have you tried powering the PSU with just the power board connected? no beaglebone and no big power connection? if its still not firing up then try the board thats having problems in place of the board that still works, eliminate everything else individually, will the board hash? if yes will both PSUs power it up with a known good power board? (dont forget to test with the terminals connected both ways just to be safe) if yes then its almost certainly the PSU, i think its likely to be the powerboard or mainboard though id guess as you still have power in the supply or the lights at the back wouldnt be on