Maybe have two different physic properties to the raft that goes with previously stated difficulty ratings. On the lower two difficulties have the static version of the raft, but on the higher difficulties have a raft that need some sort of floating device that would be used to cancel out heavy sides.
Example: Put literally everything on the left side of the raft, the raft becomes heavy on the left, and starts to sink in, depending on what the player does the raft's left side could... I don't know snap off? or maybe it causes some sort of game over (player falls into water and cannot get back on the sinking raft, which then has the player become shark food?)
Charatcer customization would be cool to see as well.
Just brainstorming. Anyway loving the way the game is coming along. Keep up the work!