In an arguably poor-received way, Dexter ended in a very odd way. Our favourite serial killer is now a lumberjack in wintery... wherever. The question comes back to why? Why does Dexter choose to seclude himself in such a fashion when everything was bumpy, but working out well enough?
Because Dexter now realizes that he is the one who is responsible for Harry's suicide, Deb's condition of being on life support, Rita's death, and Vogel's death. He can't bare it anymore. He now thinks that everyone he is comes into contact with will be destroyed the same way — that's why after killing The Brain Surgeon he stopped Deb's life support system to free her from anymore pain and even decided to commit suicide too, leaving Harrison in the care of Hannah McKay.
Somehow, he survives and shows up as a lumberjack. This means he survived, but still didn't want to go to Hannah or Harrison in order to save them from his darkness.
The last sight of Dexter eyes were still very arguable making a point for an open end.
Dexter does deeply care for Debra. She ends up paying for everything evil Dexter does. Rita has already paid with her life because Dexter gets entangled with the Trinity Killer. He originally plans on running away from everything and start his life new with Hannah or Harrison. Before he could execute his plan, Debra ends up in a Coma for life.
Dexter trusts that Hannah will take care of Harrison and his kid's future is in safe custody. The two of them - Hannah and Harrison will go away to a new place and start life new.
Dexter realizes that if he joins Hannah and Harrison he will only be putting the two of them in danger like he put Rita and Debra. If he stays alive in the eyes of the world, they will constantly try and reach out to him. He plans to fake his death. Drives off his boat into a storm, I suppose he jumped out before getting caught in the storm and got to shore somehow.
Now to the world he's dead. He can live in isolation and not endanger anybody but himself.
Why lumberjack? Well, it's just a profession that doesn't need a college degree and can be picked up based on his physical strength. This way no one gets to know that he was once a blood spatter expert and worked for the police department. The idea is that he has completely given up his previous career and the people in his life to stay as a loner.