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Ah, Utopia island. The promised land, and pretty much where your journey in Raft has been leading to this entire time. Instead of it being an ocean paradise, however, it's full of traps, hyenas, and a rather deranged inhabitant with a penchant for grenades.
The island itself is split into two main parts; the docks and the tower. The main aim here is gaining access to the gate leading up to the tower, but before you come close to that you'll need to find your way around some digging, a deceptively long water puzzle, and some... box stacking?
Finding Detto's Codes
The main accessible structure in the dock area is Detto's house. If you peek through the cracks in the locked door, you'll see that it's a workshop of sorts, with equipment strewn about the place and a rather large rocket dominating the room. Gaining access to this area involves you digging up three codes that the owner has buried around the soil area of the dock.
Outside the workshop, you'll notice a note on the table, a shovel, and a map pinned to the wall. Figuring this out isn't too difficult; you have to find the corresponding dirt mound marked on the map and dig it up. If you've navigated successfully then you'll get a piece of the code. Once all three pieces of code are in your inventory, you'll be able to open the door.
If you lose track of where the notes are hidden, simply open your notebook to take another peek at the map.
Once you have the three notes and are able to unlock the door, head inside. You'll find a harpoon and the blueprint for a big backpack.
Digging up the incorrect dirt pile will result in a lot of insects. These will damage you if you stay near them.
How To Power The Water Pump
There is a cluster of buildings near Detto's house. If you head towards them, you'll notice a water pump that is currently without power. There are five electrical cables located in and around these buildings you need to collect. Be wary that not all of these cables are accessible from the ground, so you're going to need to utilise the many boxes on the docks to gain a height advantage. Stack them on top of each other and create a makeshift staircase.
The hyenas that patrol the area will attack you on sight. They have a nasty habit of dragging you along the ground, leaving you unable to move. The only way to break this hold is to attack them, so many sure you have a weapon in your inventory and ready to use.
Once you have all the cables, it's time to connect the generator to the water pump. You'll have to use the electrical posts on the buildings in order for this to work. These cables only stretch so far, so each time you start dragging one it should be fairly obvious where it needs to be heading. With each cable set up and the water pump running as it should be, it's time to connect that to the water tank.
How To Fill Up The Water Tank
If you check Detto's map, you'll notice that there is a water tank marked in the top left corner. Climbing atop this you'll notice an item inside, slightly out of reach. To get the item, you're going to have to raise the water level inside the container so it rises to the top.
Looking around, you'll see a long hose running across the entirety of the soil area. This is going to supply you with the water needed to obtain the item inside the container, only it's currently not carrying anything. The hose itself is broken in three areas, all contained inside these circular cages.
To solve this puzzle, you need to rotate each of the three sections of the cage so it lines up with the inflow and outflow of the hose. There is only one way each cage can be rotated in order for everything to connect; a little trial and error on your part may be in order, but it isn't too difficult to figure out.
When the water is flowing it's time to climb the water tank again and grab your prize: a carbon dioxide canister.
You'll need a zipline tool for this next part. If you don't currently have one, head back to your raft and get crafting.
How To Get Into The Tower
Once you have both the harpoon and the carbon dioxide canister in your inventory, head up the starts around the outside of Detto's house. You'll come to an open room with a harpoon launcher aimed at the building opposite. Insert both key items and the launcher will fire.
Use the newly-created zipline to access the building opposite, where you'll find an entrance key on the table near the elevator. Head back down to the main dock area, to the large building where you'll find a second entrance key laying on the steps. Using the new two entrance keys on the gate you now have access to the tower, finally get to face Olof, and rescue the inhabitants of Utopia.