Lorenzo, if you make a hammer then right-click while holding it, you'll see a bunch of things you can build, including stairs and pillars to hold up more flooring.
Guason
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On the subject of pirates, you could make it so that they only start appearing once you have a couple of chests. They come to steal your chests and you have to fend them off. If they were to use, say, a harpoon gun or a fishhook like yours to grab the chests, you could cut the ropes with your spear or the axe. You could protect the chests with walls - actually having a purpose for walls would be nice!
Regarding pets, I could see having a seagull land on the raft from time to time with something in its beak. You could feed it some fish and tell it to go collect something specific you're short on. Obviously, they couldn't carry a barrel but I could see them grabbing some scrap or seeds or a tin can. Yeah, that requires a bit more intelligence than is really realistic for a bird but this is a video game.
A parrot could be a fun animal companion. While it may not be helpful in terms of collecting stuff, it could squawk when the shark approaches or give you occasional hints.