I was okay with the ending too. Two things I'm a bit critical about.
1. Although Dexter was closely tied to the ocean, he used it to dump his "trash," so wasn't very happy that he dumped Deb there. I know that's not the way he thought of her, and i knew he wouldn't leave her to be a vegetable, but I don't know. He also threw his phone in at the same spot. Okay, he was done with the contact. But couldn't they use the GPS on his phone to pinpoint that location? And find Deb, which what purpose did it serve to go through all that if they could find her through cell phone. Do cell phones even have GPS?
2. Why did he leave Harrison with Hannah? He trusted her. She loved Harrison, but she is a fugitive. When the PI wakes up from his horse tranquilizer, he's going to be able to say that Hannah is traveling with Harrison. No one is "alive" to say that he was given into her custody freely. It's harder for her to stay hidden in the world with a little boy in tow.
The fact that Dexter was still alive, I'm guessing in the pacific northwest lumber industry, it leaves that door open to a future revisit to him. After a few years, I would be ready.