Anime involving astronauts training underwater All I can remember from this is that in one episode, they are doing astronaut training. I believe it was underwater and that they have a task to complete in a specific amount of time. I also remember having a part where they are put in groups (in an isolated 'pod') and if one team member quits, the whole group fails. Also, when they are isolated, the examiners start a leak in oxygen so the students lower the temperature and 'hibernate'.
Episode 20 of Planetes features an isolation test and they have to build a model of the ship that will take them to Jupiter:







Additionally, during that episode, in order to test the isolation adaptability of the candidates, they turned off the oxygen and watched as team after team hit their emergency buttons without having a way to cope



And the way the main character's team dealt with the issue was turn down the temperature so that they use less oxygen: