Why deep ocean didn't freeze during snowball Earth?
At least four things combined to prevent solid freeze-up during Ice/Slush Ball Earth periods:   


It takes a lower temperature to freeze water under pressure. Deep sea pressures are enormous.   
Ice floats. If it sank and new ice kept forming at surface, the seas might fill with ice.   
The surface freezes first and acts as an approx zero-degree C barrier, slowing the progress of the ice penetration.   
Thermal heat from the ocean floor at hotspots, island arcs, subduction zones, and mid-oceanic rifts is substantial, especially locally.