Why did water come into USAirways Flight 1549 if an airplane is sealed and pressurized?
Two reasons:

Airplanes are not completely, perfectly hermetically sealed. They are pressurized using positive air pressure from the engine bleed air and packs. If there is a failure in the system, the aircraft internal pressure will return to the outside ambient pressure. Air will escape the aircraft or enter the aircraft if the outside ambient pressure is greater. Water will react the same way. Especially when you have the combined greater pressure of the water and the atmosphere above it. 

Combine this with the structural damage caused by the dynamic pressure of the aircraft impacting the water, the aircraft will have ways of water entering the aircraft through burst seams. The structural and hydrodynamic load alone, created by the rapid deceleration of the aircraft will be enough to tear apart the airframe, open sealed doors and bulkheads, and blow out windows.