Most of the world's major cities are not under water AFAIK.

No, but most of the largest cities are coastal, and you are somewhat more likely to get into a harbour with a hostile sub than with a hostile warship.

I really doubt that. I can't imagine that the US and the UK for example, don't have sonar networks and other means to detect subs. It might be nearly impossible to track a boomer in open ocean, but they're not designed for littoral operations and sonar networks are much more effective there. You also have to cope with an airforce and patrolling navy.

All in all, you're mostly likely to nuke the nearby ocean, rendering it mostly a radiological/environmental weapon. When you consider that you can far more easily launch a combined 100MT in smaller payloads, with a higher resulting degree of destruction than a single 100MT warhead...

I'm sure they could detect the sub, but the also have to be able to identify it as hostile and act before it can reach the city, which can take much longer. I also wonder if it's possible to "hide" under a cargo ship to escape detection?

The key phrase being "somewhat more likely".

I'm not arguing effectiveness. I was arguing against the "not under water" bit, after the person you responded to suggested a sub over a ship.

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.

Maybe, but I was not discussing whether or not it was practical.

In warfare it's only the practical things that really matter. You might attach a grenade launcher, but you don't give people double barreled rifles.