I never understood that. I don't think the movie is clear exactly what happens, but he... does swim back, right? Or does he just flounder and rely on his brother bringing him back?
Also, he quite likely _does_ die in space. Which figuratively but very directly represents not leaving anything for the swim back. There is room to square the main message of the movie about transcending limits with Gore Vidal's line (something like) "No one exceeds his potential... It would simply mean we had failed to accurately assess it in the first place." Maybe he makes it back, maybe he doesn't, but he goes, does what he sets out to do with no consideration of whether he will be able to return and then tries to return. It's exactly equivalent to people who say they would go to Mars even without knowing whether it is a one-way trip at the outset.
One message of the movie could be that there is no fixed measure of potential. There are too many variables. One of them being how badly someone wants to achieve something. Thus it’s massively unfair to measure someone’s potential at birth.
He is willing to die to succeed. If that meant in that moment to outswim his brother he had to drown then he was willing to do it. His brother was limited by his fear of death and his subconscious need to still turn around and swim back. He had no concern so was willing to go as long as it took. It just so happened that he was physically fit enough to be able to make the return swim as his brother quit in time.
He didn’t limit himself planning for the future, rather he found that when the time came he had strength he didn’t realize he had.
The movie leaves it a little vague, but his brother disappearing would wrap up the plot nicely, as would his brother finally seeing the Vincent's perspective and keeping his mouth shut. On a broader level it's the "burn the ships" strategy; nothing motivates like having no alternative.
The one I didn't really understand was "It's too late for that. We're closer to the other side."
I thought it was a large lake or inlet, and it would take less effort to keep swimming to the land at the other side than it would to turn around and swim back to the start.
Well that would make sense. I thought it was the Atlantic or Pacific Ocean. Especially since Anton seems confused by that also. Also if they were closer to the other side physically, rather than metaphorically, why turn around?