Continuity mistake: After the rope keeping Ryan and Matt tethered together is snapped, Ryan continues drifting downwards towards the Earth. The shot continues panning until the camera stops with the camera high up. As she falls, the ropes from the parachute around the satellite are on Ryan's right. In the next shot from low down, the ropes from the parachute have moved to Ryan's left. She was only shown spiralling downwards throughout the shot, so her position next to the ropes would not have been affected. (00:30:25)
Answer: Swimming does not have the same gravity related constraints that walking on land has. It is not until she is on land where she shows signs of facing difficulties with the Earth's gravity. Also, when she swims up to the surface, she is rushing so she doesn't drown and in doing so, uses up most of her energy because she has been in space and is only now readjusting to Earth's atmosphere, so when she is above water and swimming over to land, she visibly shows signs of being exhausted and out of breath as she used up most of her limited energy attempting to swim up to the surface.
Casual Person
Swimming still has gravity related constraints, though right?
Gravity pulls water towards the earth, yes. But for a swimmer, the water provides buoyancy and supports them. The closest thing you can come to weightlessness on Earth (not including the flight training where they take you into a plane that glows up then drops) is in water, because it floats you.
She is swimming up to the surface at the fastest speed she can, so she doesn't drown. Perhaps there are some gravity related constraints to her swimming, but she is trying to fight against it so she can get to the surface. When she is above the surface and swimming/floating back to the shore, she is visibly exhausted, so it is apparent that she used up most of her energy in trying to fight against the gravity related constraints.
Casual Person