What Can I Do To Finish Redbeard's Booty?

I've been able to complete stage 4, but for some reason when stage 5 ends the letters:
"A key is laying on the ground, it seems there were more than one. Better take it and figure out a way off the island." and "Maybe you can fix this boat and get off the island?" won't appear.
Once i get the key (I have 3 now) and build the boat,, it simply wont let me off the island.
The boat despawns and the tag to build the boat reappears.
/kill and redoing the quest also doesn't work.
If i use /kill and go talk to Marston to supposingly complete the quest, he doesn't respond.
Does anyone have any suggestions?