I'm legitimately worried Hardy actually dies in the first 5 mins and that Branagh is only seen at the beginning of the film. That'll be a shame if that's the case.
Has this prologue been confirmed to be the first 5 mins? It seems like it's cut differently, like it could be a condensed version of the first 10 mins of film or something.
I don't think it's the first five minutes. Unless there's a flashback, surely some of the scenes in the trailer happen before the prologue.
Some of the characters will die; it's inevitable.
Whether or not it's Hardy remains to be seen.
So far they hinted at a Hardy death and a Harry death in the trailer and prologue. I don't think we saw Hardy on the set, so it seems unlikely that he ends up on the beach with the other stranded guys. I guess it's possible that his death happens sooner rather than later.
I don't think Harry's character drowns, at least not like it's shown in the trailer, unless Nolan shot multiple scenes to confuse people months in advance:
Tom GC's character is putting one of the life jackets you see in the prologue on Harry, and in the scene in which Harry 'drowns', he doesn't have it. He has the standard army life jacket (which he also has on the beach). I mean, he could lose the orange life jacket or take it off at any given time, but to me, it looks like the scene with Tom GC will come later. Maybe they even rescue him when he's about to drown.
PS I added a spoiler in your text because it was a bit... spoiler-y lol
