It is probably worthwhile noting that elt beacons are not the same as the underwater beacons attached to the CVR/FDR or similar. The former has a totally different frequency range (not sure about its water penetration and a quick search was shy to talk about max depths) and a much shorter battery life I.e.
http://v5.books.elsevier.com/booksca...0750681377.pdf and the underwater beacons can work for over a month at up to 6 km deep (if that is the rating the reality might be better)
http://rjeint.com/pdf/DK120.PDF plus the methods of activation are different. I don't know how rugged they are, if there was a colossal explosion or impact in flight, as to whether they would survive no matter what the spec sheets may imply.