I see lots of suggestions about day/night cycles (which I know are tough to create), and I see a few suggestions of beds. To add to both of those, I'd suggest two types of beds to start with: a hammock and a palm frond sleeping mat. More options could be added for types of beds later on.
Certain beds would cost more but help the player recover more health (in the event of injury from the shark), players can only sleep if their injuries are severe or the sun is setting. Players also must have decent hunger/thirst levels before sleeping, as sleeping will make these go down. If a player is injured to the point they must sleep, they will wake back up when too hungry or too thirsty.
While sleeping, the shark might destroy 1-2 foundation tiles. (As per the difficulty level suggestions, this could be increased with an increase in difficulty.)