Matt Kowalski: Half of North America just lost their Facebook.
Ryan Stone: It's time to stop driving. It's time to go home.
Ryan Stone: It's starting to get hot in here. The way I see it, there are only two possible outcomes. Either I make it down there in one piece and I have one hell of a story to tell! Or I burn up in the next ten minutes. Either way, whichever way... No harm, no foul! Because either way, it's going to be one hell of a ride! I'm ready.
Matt Kowalski: I know I'm devastatingly good looking but you gotta stop staring at me.
Matt Kowalski: Houston, I have a bad feeling about this mission.
Mission Control: Please elaborate.
Matt Kowalski: Well, it reminds of a story.
Ryan Stone: Clear skies with a chance of satellite debris.
Ryan Stone: I hate space!
Matt Kowalski: You have to learn to let go.
Ryan Stone: Fuck!
Matt Kowalski: Copy that.
Ryan Stone: You're losing the altitude, Tiangong. You keep dropping and you're going to kiss the atmosphere, but not without me because you're my last ride. Wait... [Reaches for a fire extinguisher] Five. Four. Three. No more just driving. Let's go home.
Ryan Stone: Hey, Matt. Since I had to listen to endless hours of your storytelling this week, I need you to do me a favour. You are going to see a little girl with brown hair, very messy, lots of knots and she doesn't like to brush it and that's OK. Her name is Sarah. Can you please tell her that mama found her red shoe. She was so worried about that shoe, Matt and it was just right under the bed. Give her a big hug and a big kiss from me and tell her mama misses her. You tell her that she is my angel, and she makes me so proud. So, so proud. And you tell her that I'm not quitting. You tell her that I love her, Matt. You tell her that I love her so much. Can you do that for me? Roger that. Here we go.
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