My XP3 seems to be leaking from the hole for the power cord. I am thinking about hot gluing it to seal it. but if it is a major defect I should probably return it.
If it's fairly new I'd take it back to the store, if it's a bit older I'd contact the company. Either way it seems that it is something that you shouldn't ever have to worry about. I also wonder about the safety of running it. The cord isn't meant to be submersed in water, so I don't know that I would continue to run it, if it were me.
The casing should not have water inside where the motor is. If you recently washed the filter, it could be water trapped in it but if it is leaking during operation then you should return it. Check to see if there's replacement coverage from where you bought (Drs Foster Smith has a 3 month exchange policy) or check the see what's Rena's warranty says.