She was on a ship? I really doubt they would keep waterbenders on a ship. Considering they had her for what seemed like years they almost definitely had her in some prison far away from any water.
As for the question... One person escaping is a lot easier than a dozen people escaping. You also have to realize that all of those other waterbenders were completely broken, tortured for years. Hama was the most recent victim, everyone else was there for longer than she was.
She also became jaded and defeated. Maybe she considered them weak and not worth saving.
I feel like this is a similar situation as when people complain "why didn't the earthbenders just escape when Katara gave her speech in episode 6" Concentration camps are no joke.