Triskel:
Mr Snuggles - thank you for posting link to hi-res pics. From the apparent concave folding of the skin, the pressure hull was probably intact on entering the water (otherwise the water pressure would equalise inside and out). The hull probably broke on contact with the sea floor or perhaps on the way down (if the water was stratified, on encountering layers of different density)?
Did you look at the right on the highres, just above the hyphen? (It needs to be the picture with the A upside down.) To me it looks like something round hit the airplane - or I need to adjust my medication... *joke* Maybe, just maybe they hit hail of some kind...? Absolutely not explanation in and of itself but might give a hint, possibly?