!BULL SHIT!!!...
For a bubble of methane to be mixed at the right proportion (an explosive mixture) and large enough to go 'boom' instead of whump (burning, but not exploding) and be large enough to be an ELE, would be highly unusual to say the least. Just the atmospheric mixing required is phenomenal.
As for floating around dissolved in seawater, the closer it gets to the surface, the less soluable it is, and methane is really pretty much insoluable in water at atmospheric pressure.