There are pockets of methane hydrate that forms a solid under high pressure and low temperature at the seafloor.
It however sublimates easily into a gas that rises to the surface. If you release enough of it at the same time, the bubbles destroy a ship's floating ability, making it capsize. Fast. With very little reaction room.
BTW, to destory an entire fleet using this method is nigh impossible with six torpedoes, even with a methane hydrate facility underwater. Also, once it had destroyed the fleet, it probably would have created a massive tsunami headed right for Area 11.