Just to calrify something else, as well. The methane gas theory has nothing to do with methane burning, or rotting plants. It has to do with methane hydrates (I think, hydrides, maybe? Chemists? Geologists?), which are solid, unstable methane-containing compunds on the sea floor, Slight disturbance can cause them to release the methane, sometimes in large amounts. This causes bubbles to rise in a big group, and a bubbly ocean produces less bouyant force than calmer water.
Still very unlikely, as a ship would have to be right over a hydrate area, at the exact time that a tectonic rumble set it off. Logically, there'd be numerous sightings of bubble rises (near misses) to every dissappearing ship...and we don't have that. It's highly unlikely, to say the least.