Does Anyone Else Think...
That the show had two different endings shot...one just in case they didnt get renewed...in which the Master does get dumped into the bottom of the ocean and Zach unties with Eph after he kills Zach's mother. But since it did get renewed they went with the other ending which leaves it open for another final season.
This would be the reason why the ending of this episode seemed a bit forced (letting the Master get away)