Adding an end game or another objective other than just surviving and avoiding the shark, like an eventual rescue perhaps.
Adding more solid walls, floors, roofing. "customisation" type stuff.
Add fish to spear possibly, as well as adding some other fish to come off the rod.
Adding more materials, to diversify the crafting as well as give the possibility of other items like rubber and plastic leading to things like a rubber boat: for the following suggestion.
Adding islands to swim to/paddle to, perhaps finding other materials to build things out of.
New breeds of shark/quantity of sharks.
More ruthless shark nature, notices you are not stabbing it to keep it away and attacks more frequently and vice versa.
Birds, add some birds, surely there are some seagulls or something wanting to steal your coconuts?
Temperature bar, all that cold water and no warm place to stay must be making you sick, right?
Clothing, even just a seaweed shirt and pants to protect from the sun, or a shark bite.
Night time/day time cycle.
Furniture, more specifically a bed for the night time cycle.
Pets, i want a pet seagull or a pet crab or a dolphin friend to help keep the shark away.... or even a shark to keep the sharks away?
Random items that could have washed out to sea, a floaty, a tennis ball, plastic bottles.
All just suggestions, but theres a large potential for this game and it takes one or two good ideas to make a good game better.