Because not enough methane! In water, it makes a huge difference, because ships are WAY heavier than methane gas (thus would sink before they could displace a similar mass). Airplanes can still fly through the stuff, assuming their electronics aren't messed up by it, but boats can't float through an upwelling.

This is the great danger lurking beneath the Caribbean (http://www.globalresearch.ca/doomsday-methane-bubble-rupture-how-the-bp-gulf-disaster-may-have-triggered-a-world-killing-event/)....