@LACHESIS & @EMERALD STAR
"Aight. I'm going in. I'm kind of expecting an ambush, but that's my gut, not a cape thing. If I get to them safely, I'm gonna try to get even some temporary safety, then slow us all down to give everyone else time for a rescue before the air supply runs out. Don't count on it until you hear me, though." Cassandra fumbled with the mask for a few seconds, not only less familiar with them but holding herself in place with an elbow hooked over the railing. She got there, though.
And then she took off. Her bubble of accelerated flow was unmistakable: the water piled up in front of it and collapsed behind, until the flow stabilized into a wave rolling past her. It was a rare sight; her powers were almost never this visible. There wasn't much opportunity to appreciate the display though, since she was moving a few hundred miles per hour. She vanished down the stairway in a flash and maintained the pace down the hallway below, according to Liz's display.
If someone else could have properly experienced Cassandra's perceptions, they might have understood why she complained so much about the swim. The world stretched out ahead of her, in the direction of the flow of water, but surfaces were incomplete and disconnected, made of only what she might brush against in passing. All around her, though, it closed in until there was nothing at all beyond her armspan. If she drifted right into the center of the hall, the walls didn't even exist until several feet ahead. Behind her, only an empty void. At least she could claim she was shivering because of the cold.
Really just using Speed here. Call it 7 ranks at most, because she's fighting the environment so much.