He tried to swim, and it was quite difficult to do with his condition still in his special outfit. Maybe it would be better to just take it off. However, Yu refused to do so because he didn't know for sure if he was really on Earth, or another planet he didn't know about.
Of course, taking off his astronaut outfit at this point was not a wise choice. What if it turns out that this is a different planet, and the conditions outside don't allow humans to breathe?
Yu didn't want to take that risk, he kept trying to swim to reach land as quickly as possible even though with great difficulty.
In the midst of uncertain conditions still unfamiliar to him, Yu suddenly felt anxious, something might be stalking me, he thought.
And he gasped and was terrified because he saw a huge shadow beneath him. He stared, looking like it was some kind of large predatory fish, and he quickly swam back, trying to get away as fast as he could.
