Perhaps a day/night and weather cycle. eg. night its more dangerous and bad weather you 'get ill' from being outside in the cold and rain too long and a way to get around that without having to build a hut of sorts would be to have a small fire. And in really sunny days your need for water goes down faster. Also make it harder so it takes to to get a comfortable, big raft. and to let all the needs go down slower in normal times