To be fair they kept her trapped in that mirror for 5000 yeas despite knowing she was in there, and in the end she opened up to Steven and thanked him for fixing her gem.

I don't think she tried drowning them, just fighting the gems with those water-clones and the hand thing from the start. She was justifiably pissed for being trapped like that for so long.

Plus, I'm not sure if she "sold them out". She might have been forced to tell them what happened after she got back.

Though, they do make it pretty clear she doesn't give a damn about anyone else but Steven, and only because he helped free and heal her before.