The nuclear reactor that is powering the sub is quite small, producing generally 1/10 of the energy of a nuclear plant. They are designed to withstand direct torpedo hits, and historically when for example a nuclear sub had most of its own torpedoes explode from within the sub, killing everyone inside, the power plant itself survived without damage (there exist is a good documentary about kursk accident and the salvage operation of its two nuclear plants).
Even if it did have a meltdown, it is likely to be a depth if its hitting an underwater mountain. You could explode the biggest nuclear bomb every created at the Mariana Trench and the only effect we would get at the surface would be some bubbles. Water is pretty good at not be compressible, while at the same time able to diffuse the explosion while it is traveling upwards.
The problem of a nuclear sub colliding underwater is the human lives lost because of the crash.