There are signs that the Challenger crew was conscious for at least a short time after the explosion. The most prominent evidence is that one or two of the emergency oxygen packs were activated and that had to happen manually.

The Challenger had a significant vertical velocity when the explosion happened. If memory serves, the explosion happened at something like 40,000 feet. Radar evidence indicates the crew compartment continued upward to at least 60,000 feet. It took several minutes before the compartment struck the water.

The Challenger crew didn't appear to be wearing pressure suits when they walked out to the Shuttle. Odds are the crew compartment lost cabin pressure in the explosion. If that's the case, the crew would've lost consciousness within a 30-60 seconds. When you get much above 40,000 feet, oxygen alone isn't enough. You have to have pressure on your body, either by a pressure suit or a pressurized cabin.

If I'm right about the crew compartment losing pressure, then the crew would've passed out in less than a minute. At least, I prefer to think they weren't awake all the way down. The compartment hit the water doing over 200 MPH. The deceleration would've been so high that their bodies were ripped apart by the g forces.

Since then, all crews have worn pressure suits and parachutes during launches and reentries. In the case of Columbia, the crew compartment was probably torn apart by the heat and aerodynamic forces. If the crew were exposed to the Mach 18 blast, they would've been ripped apart very quickly. Consider this - if the forces at that speed were strong enough to rip apart the Shuttle, then a human body (even in a pressure suit) wouldn't stand a chance.