My old horse had Shires Highlander rugs which were cheap and chearful, they were 600D and I never had an issue with them leaking. I now have Rambos, which are Ballistic Nylon, ok much more expensive but absolutely fantastic, the water seems to bead on top of them and they dry very quick. I wouldnt go back to the others, Im on my 4th year and it still looks great and that was with my horses friend swinging off it at every opportunity.
The denier of the rug has little to do with how waterproof it is. The denier is a sign of 'toughness' & its resistance to being destroyed on wire, ripped by other horses etc.
As Toby says, the denier is only the weight and thickness of the fabric. Whether it is waterproof will depend on the quality of the manufacturer and the age of the rug. After a few washes, if you don't have it reproofed it will start to leak.
I have 2 of the Amigo Lites that have a 600d, they are waterproof until it rains really heavily, then they really start to leak. I would try and buy one with a tougher outer to be honest, would be worth it in the long run.