There seems to be some chatter on the internet to this end, where an eruption of methane gas could cause a tidal wave of 100 meters in height. There are two articles I found about this, both of them apparently originating from the Helium website: http://www.helium.com/items/1868691-...-kill-millions
I beleive one of Alex Jones radio guests also talked about this scenario, as did Dr. Bill Deagle with our Bill, however claimed it would be a build up of steam that causes the eruption.
These three stories emerged after the interview by Jones talking with Lindsey Williams and the broken casings because of the high pressure that the undersea oil is producing coming to the 'surface'.
Are the three sources scaremongers? We won't really know until it happens, and also of course this sort of story would do a great boost for property prices of the region.
From the sources I like to get my information from, there seems to be more of a concern about toxicity and polution rather than an actual tsunami. But now the subject has been brought up there may be more in the can of worms that meets the eye.