If it hits the sea out of control or if you fly it into the sea, you'll still get wreckage. Water is at least as hard as concrete at >60mph and I very much doubt they tried to ditch, since the visibilty was poor. Even if they did, the lack of ability to judge altitude above water (depending on water state) could have easily caused the a/c to hit the water and fragment.
Structural failure is probably doubtful, unless down to a preexisting fatigue crack etc - it's rare that turbulence gets so bad in the UK/IE, unless you fly into a CB at well above Va (taking into account ultimate loads etc) The only manouver that would cause structural failure (besides aerobatics) over the sea would be a spiral dive.
Anyway let's leave the cause to the experts, although god knows it's tempting to assume.