I have spent 9 years at sea , so know what i am on about. Swells can be very high even without winds. The perfectly calm sea from where the showcase test of K-15 was recorded is only ideal situation not even normal at high seas.
Launch can be done from harbors and coastal bays where similar conditions exist ,same as chinese used to do with their original Ballistic missile subs which stayed near coastal waters.
Great to know that you had spend 9 years at sea. BTW did you spend this 9 years on sea or in sea?
Anyway for a basic submariner, it is a known fact that even a typical 10 feet swell on sea surface doesn't effect the normal functioning of a submarine. Anyway launching a missile is not a normal day to day function of any submarine, but neither its away from norm.
@airtel had already refuted the claim of it being a finned structure, so no point of stating that again. On other hand, you might know the fact that K4 has already been tested from 30m depth or 90 feet of depth. In other words from the depth of a 9 story building. Now at that depth even at a category 5 hurricane, sea would be relatively calm then the surface. BTW what you said about the launching is partially true. It would be a problem to launch cruise missiles in such a weather as they are launched from relatively shallow depth and from the torpedo tube. But an SLBM is launched from vertical launch tube and hence would face a lot less turbulence.
Now in an SLBM if we talk about the launching phase, at first a gas generator kicks in to lift the missile out from the silo to the surface and after that the rocket motor kicks in. Now you could say that in a swelling sea it would be a problem due to the gale force. Yes, firing a 3 ton CM at the sea surface would be a problem, but firing a 19 ton SLBM is totally another thing. The force which the rocket motor would generate is enough to break a depth of 10m for any SLBM. Moreover K4 being an IRBM has a range of 3000km. Means you don't need to be near tropic to launch it and hence would be out of the most violent of sea storm. Launching it from 2000 km away from shore would mitigate lots of turbulence.
Moreover you should know the fact that it is more China and Pakistan specific. Means we would be launching it from either Indian Ocean, Arabian Sea, Bay of Bengal or from South China Sea. Now in these regions we do have cyclones, but not have so much of turbulence like that of Pacific or Arctic. Yes A-6 with 10K+ range would be launched from Pacific and Arctic for sure and then we would solve these issues too. K15 had been a technology demonstrator for SLBM and most of the issues had been resolved with it.