Andrew, that is a great point. A very small and controlled nuclear explosion may or may not allow radiation to reach areas populated by humans but what about those who take to the seas? What about non-human life in the area?
That said, another issue raised in the discussion I heard on the radio would be to use a non-nuclear tactical bomb instead of a nuclear bomb (raised due to the concern of radiation). Even that option was deemed too risky to try due to the geological reasons I mentioned. You could end up with multiple uncontrollable leaks directly from the sea floor. Now, at least it's a single leak from a pipeline and there is still hope that the pipe can be sealed.
Now, assuming these relief wells fail, would it be worth it to consider a small preferably non-nuclear detonation to try to pinch off the well? With no viable alternative, it might be the last option available. Even if this were the scenario, though, would it be worth the risk of making the flow significantly higher with no way to attempt to capture it?