But at that point ~1/2 your detonation is over the ocean and useless. Further, unless you surface the water will dramatically reduce the bombs effectiveness.
You're still going to do a lot better than approaching with a ship that is plainly visible to anyone, and which will be taken out before it gets anywhere close, was the point.
Sure, but an ICBM with a single 10MT detonation would be dramatically better than both as you want it up in the air and over the population center. Remember the closet km is always gone, it's a question of what appens further out that's important. And thus you don't want a line of buildings to protect anything.
If your target is the city/population. If it's the president then a surprise attack would be better, although I doubt a nuke carrying enemy sub would be able to get that close.
Peak speed for an ICBM is in the ballpark of 6-7km/s (any faster and the payload would go orbital), and it takes about 10 minutes to accelerate to that speed. That's New York to Moscow in around 20 minutes.
So, a sub 1,000 miles from shore can hit you faster than you can get out of the blast range when you include a modest detection > reaction delay unless your sitting in an airport. It's vaguely possible you could get into a bunker in that time period, but unless you sleep in a bunker even that's not really viable.
However, by setting up things to respond in the event of a nuclear attack it does not really matter if you take out DC / Moscow as the other subs can respond anyway.
