Yes Diurex water pills may help to get the marijuana out of your system but it depends on how long ago you smoked as it can remain in your system for some time.
Marijuana takes between 3 and 30 days to leave your system, depending on how often you smoke. If you smoke excessively it could take longer, up to 90 days.
The best thing I can advise is to drink two large glasses of cranberry juice, I am not totally sure if this will help, as there are mixed feelings on this, but it certainly won't do any harm and is worth a try. After that try the Diurex water pills and drink lots and lots of water.
Work out as much as you can, sweat it out, keep drinking water and go to the loo as much as possible. You need to really push it through your system as quickly as possible so drink as much water as you possibly can.
I hope that works, good luck!