The pipeline is 80m underwater - a ship could be sailing right over and not even realize an explosion happened under it.

What about the methane? How would it interact with the ship if it was directly atop the leak?

Sure: when a large amount of gas is released to bubble up, the average density of the gas/water mix can be too low for the maximum displacement of a ship passing above to carry it's weight. Ship drops beneath the surface until it hits pure water or a mix close enough, and most ships aren't built to recover from that.

Or, the ship breaks its keel because the front and back are supported, but the middle loses buoyancy.