So, our favorite serial killer turns his boat into an oncoming hurricane and then apparently swims ashore. I have seen some reference to the gulf stream being 5-7 miles out from Palm Beach - which might even be an underestimate since we don't know exactly where he was. Doesn't it strain credulity that he could swim such a distance in the middle of a hurricane? People compete in such events (10 km swims or more - 6.2 miles), but we've never SEEN him swim - are we to believe he could really have done this?
In an interview with Executive Producer Scott Buck he explains that it is mentioned in a different episode that Dexter had an emergency life raft on the Slice of Life, implying that the raft is how Dexter got back to land. The full interview can be viewed on the Hollywood Reporter website.
Earlier in the season, Dexter was showing his boat to a prospective buyer. He gave a rundown of the features of the boat, which included some sort of emergency life raft. So he likely used that raft to escape once he'd put the boat in the path of the hurricane.