Quite likely to be busted. Had you turned it off before the water hit it, you could probably just have let it dry out (evaporate) and the amp would then probably have worked. But now it sounds like something is shorted/burnt/broken...

I presume with stop working you mean pop fuse / go into protection mode or something - if you get sound with only one speaker wire hooked up you probably have one of the leads of the speaker wire shorted to ground somewhere... Could also be internal short on those 3 non-functional outputs that has killed them and makes the amp dislike getting power from the good terminal into those...

/Michael