Frankly, people these days equate storytelling with reality waaaayyyy too much. Classic storytelling since the beginning of man has almost never strived to be realistic, but to be symbolic and relatable. Realism is only a recent trend within cinema that some people take way too seriously. That's why Nolan specifically talked about his Batman series never going for realism, but for verisimilitude. Seriously, if you've ever read any great novels, or classic stories, or even many great movies throughout film history, they define the world they're inhabiting and stick to those rules, but rarely are they bound by realism. I agree with batlobster that while they may have stretched the suspension of disbelief in this case, they certainly were still well within the confines of what they'd previously established.
What it means to the story is: Dexter purposefully survived the storm to punish himself by living out his life in exile rather than simply taking his own life. How he survived is not the question that should truly beg answering. It's the why. Whether or not that was at all accessible to most of the audience is a much more pertinent question to debate, and I'd say most of us agree that it's a pretty tough conclusion to arrive to for those less inclined to dig into what they were trying to say with it.
