Match 40'000.
Using 1.48 km/s as Mach 1 underwater, we get 59'200 km/s. The speed of light is 299'792 km/s, so we're now at 19.7% of the speed of light.
The kraken and its surrounding would turn into a nuclear bomb, the crust of the planet would be punctured at that location, there would be a...