Rainboe,
Submarine methane release is a fact (
UCSB Press Release: "Gas Escaping From Ocean Floor May Drive Global Warming "). Same for CO2 release from lakes (
Lake Nyos - Wikipedia, the free encyclopedia) but for a different cause.
The question in the context of this threas is not if it happens, but what could be the effect on ships and planes. Some evidence suggests that it can sink ships as the bubbles in the water reduce the desity of the water and alllow ships to ride lower, to the point that they are swamped.
For planes flying just above sea level these effects could have some result, but not in terms of boyancy, but in terms of preventing o2 from reaching the engines. That most certainly would be limited to a few 100 feet above sea level at most.
So, for jet transport the chance is about that of a snowballs you know where, but that does not mean the situation is totally absurd.
TME