I am surprised, how come all the posters on this thread are so naive about nuclear submarines.
Nuclear submarines are not important for mobility or lack of mobility; not necessary for a blue water navy .. or whatever reasons posted above.
The single most important reason is ..... nuclear second strike capability. Only nuclear submarines can stay under water for months at a time. And nuclear weapons are safest to hide in the ocean than anywhere on the land (silos) or mobile launchers.
If Pakistan had nuclear submarines, it would stop worrying of a surprise US operation to takeover it's nuclear weapons (e.g. a US operation like Operation Geronimo).
Further, Pakistan (and coincidentally the rest of the world too) will feel much safer, if Pakistani nukes are safely secured in nuclear submarines, where even terrorists will find impossible to reach.
When the Pakistani nukes are safely hidden in nuclear submarines, it may not actually want to further increase the size of it's nuclear arsenal.
Indeed, if China refuses .... even US or Russia (.... or actually, even India !!!!) could sell a nuclear submarine to Pakistan, so that Pakistan no longer needs to buid a land based nuclear arsenal which can fall into the hands of terrorists.
Potentially, the only advantage from China is that it may give it to Pakistan for free or friendship price (just like the 50 JF-17s deal). Pakistan could further enhance them with western avionics.
Nuclear submarines for Pakistan is good for everybody