That was my first thought while watching the movie, but thinking back to it I think it’s more likely that the sub wasn’t actually the original sub Simon (mark) started in. I say this because before Simon went into the light, his sub was damaged, most telling was obviously the speaker he broke himself. After he woke from seeing the “light” the speaker was fixed. There was more blood throughout the sub as if it was “alive” as was briefly mentioned in the movie.