Make it harder as you progress, one idea I think would be scary and cool is like a herd of whales that could bump and break multiple foundations. They wouldn't happen as often as shark attacks, but when you hear them your heart would race as you try to figure out what items your going to have to replace.
Maybe also a hardcore mode where your raft will slowly decay from the wood rotting out.
Instead of cooking salt water, maybe add in some weather effects where you need to set up some sort of water catcher.