It strikes me as a too smart by half idea on how to smuggle coke. Not being a nuke, it would have to spend most of its time surfaced, where it would stand out like a sore thumb and attract a HUGE amount of law enforcement attention. It seems to me that an ordinary yacht, of the kind people sail around all the time, would be a lot less likely to be noticed, absent a tip-off.
The subs don't make the whole trip under their own power. They are towed through international waters and then make the final run in for deliver.
To add to what SampleMan said, a sub running at that depth could get air via a snorkel. I agree, though, that if it was spotted the Coast Guard and the Navy would on it like ugly on a toad.