Finished the game today and it was fun.
I have some theories about the ending, considering the fact I heard (zombified) growling and the door knob on the boat turns quickly which leaves a huge cliffhanger. It also hints: Will there be another Dead Island?
1: The survivors were turned into hideous beasts when the 5 immune used the mutagen. After that, hell on another islands starts again.
2: The growling was probably from some zombies from another identical boat.
3: The growling was just to mess with people's minds and the survivors are fine.
4: How were survivors able to survive without food or water in six days (unless there is enough food in there to last a whole week)?
Any theories can go down in the comment section.