Is duct tape waterproof? - yes, and no. The upper and lower surfaces are relatively waterproof, but it has a fibrous mesh interior that will soak up water from the edges of the tape, and if left long enough, this will turn the tape into a sticky wadge.
Next.... To seal the water leak you need something that will press down and seal off the source of the leak. Since it isn't elastic, duct tape wound round the pipe at a connector will be full of gaps. That's not good. Instead, I'd recommend 'self amalgamating tape'. If you're not familiar with it, this is a rubbery tape that's got no adhesive. Instead, with the backing removed, the tape will weld to itself. Cut off about a foot, and stretch it to half as much again, (easily done). Wind the tape tightly onto where the leak is, and to either side, keeping it tightly stretched as you do so. If you need more, just wind more on to what you've already applied.,,
http://www.screwfix.com/p/self-amalgamating-rubber-tape-black-19mm-x-10m/87717