According to his driver's license, Dexter was born on 02/01/1971. This is the real-life birthday of Michael C. Hall.
When Travis 'Doomsday Killer' Marshall is looking at Dexter's driver's license, the last three digits are shown as "66-6".
Travis Marshall leaves the house saying "Today's the Day." This is similar to but the opposite of the common phrase Dexter has been known to use throughout the series "Tonight's the Night."
The boat that rescues Dexter from the ocean when it looks like his end is near is named the 'Milagro' which means Miracle in Spanish.
At one point Travis Marshall is seen eating an apple. The symbolism is a well-known biblical reference to having knowledge of good and evil, as is fitting with the themes of season six.