It's also a matter of stability in the water. In the Sky Shuttle incident, the passengers made no attempt to disembark once the aircraft was in the water, so the crew opened the doors from outside, which sped up the taking on of water, as you might imagine.
Training on opening the windows from outside the airframe probably wouldn't go amiss, but I think you'd need a real training device, so people would have a sense for how much force needs to be applied, and where.