Ok, so I've never read the comics and didn't see the first and I'm a girl so I might be missing something crucial here but.... (don't read this if you haven't seen the film)... why didn't the boy with the frosty hands (see don't remember his name) just freeze the tidal wave breaking through the damn and save them all. That way Jean needn't have died at all.
Or am I just ruining things for the next sequel??
Good idea about Bobby (maybe his powers weren't up to it), but basically they are setting things up for the Phoenix storyline (or a variation of it - I can't see aliens being involved) which sees Jean resurrected with greater powers, hence the phoenix-like shape glimpsed under the water at the end of the movie.
To be a complete bore, I was quite a fan of the comics at one time, in the comics and the cartoon followed a similar line, Phoenix isn't Jean Grey, it's a copy of Jean Grey, it couldn't save her so it simply took her place but years later, Jean Grey turned up living as someone else but without her powers (in the comic that is). Phoenix is actually something far more important but I won't bore you with the details.