Continuity mistake: Just after Matt retrieves Ryan, Matt tells her to set her watch timer for 90 minutes, which she then does. A shot is then shown of Ryan's arm adjusting the watch timer to 90 minutes and the time on the watch reads 00:32. Later on, when Ryan is in the Soyuz capsule, she looks at her watch and says "Seven minutes to get out of here." It is shown that the time on her watch reads 02:21 (1h 49m later) but the timer reads 7 minutes and 26 seconds left of the 90 minutes. (00:19:10 - 00:46:30)
Continuity mistake: Just as Ryan and Matt are about to bump into Shariff's body, the POV shot of Ryan shows Ryan shielding her face with her hands. It can be noticed her right hand is positioned in a shifted 10 o'clock direction. Instantly as soon as the next shot starts, her right hand is now in a 12 o'clock direction, not shifted at all. (00:20:30)
Continuity mistake: On the way to the ISS, Matt asks Ryan where's home. Ryan replies "What?" and Matt says "Down there. Mother Earth." Behind Matt as he says this is the rope keeping Matt and Ryan tethered together drifting around. At the end of the shot, the part of the rope shown in front of Ryan is shown to be drifting around differently to when it cuts to a shot of Ryan. (00:25:00)
Continuity mistake: When Ryan and Matt reach the ISS, as Ryan says "Wait, you have to brake!" the ISS is visible from the POV of Ryan. About two shots later, another shot of the ISS is from Ryan's POV, only now they are way further away from the ISS than they were two shots previously. The second shot may appear a bit confusing because it is at a different angle, but they are definitely much further in the second shot. (00:29:40)
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)
Continuity mistake: Right at the end, when Ryan's pod lands, in the beginning, there are hardly any plants, but, in the next shot, there are lots of plants; the same with land, one second she's far away from land, but then, suddenly, she's right near it. (01:21:10)
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