Story related and cant be missed
*** Spoiler - click to reveal ***once you make it to the shore the gang from Monroe (the community you just burnt down) will arrive behind you and force you to the ground. Just crawl forwards to safety, when you stop getting shot, Michonne will use her favourite trick to navigate the group through the walker herd. Just walk forward while following the button prompts. She will start to hallucinate again and as soon as you're clear you will unlock.