A few ideas ive thought of:
More weapons! (harpoon gun,different levels of spears etc...)
Doors for buildings
Different kinds of walls to make different buildings (stone house,wood house,bamboo house etc...)
Decorations for the inside of buildings such as beds,chairs,wooden desks or a bath that we can fill with water from the ocean (just for aesthetics)
More ways to catch food and different kinds of fish
Different levels of fishing rods such as basic rod(made for catching basic/small fish),medium rod (can catch bigger fish like tuna,lobsters etc... and experienced rod (for small sharks and other underwater creatures\
Different kinds of sharks as enemies that change everytime you kill the main shark (meaning): the first shark is the basic one we already have,then when we kill it it becomes a tiger shark, hammerhead shark,great white shark and finally a megalodon as a final shark
Different ways to make platforms (not just building pillars to stack houses for multiple levels)
The already mentioned idea of crafting diving gear and exploring is really interesting and the idea of crafting different kinds of rafts sounds fun too
Upvote/reply with any ideas/changes you have!