Out of the frying pan
Unfortunately with this achievement I can only give you tips and a bit of guidance. The location of every sea monster and plane is different no matter what seed number you have. Consult your cartographer map on the main menu to find their locations. Get to know your map well, (Landmarks like ships, odd shaped islands so you can get your bearings and find out where North, East, South and West are. A good tip is to take a photo or jot it down on some paper.
The requirement is to kill the 3 following sea monsters-
The Great Abaia (Eel)
Lusca the Great (Squid)
And to find and restore your plane.
Before you start, change your setting from Shark and Snakes as default > passive. Save as often as you can during the game. If you get poisoned you're best off reloading your save as finding Pipi plants to make an antidote can be a bit of a pain. Keep an eye on your water and food meters. I personally got though most of the game eating sardines which are easily caught with a crude spear. (To eat them you need to skin them and cook them). I found that 5 at a time in a smoker worked best. I also farmed Fibrous leaves as my water source using the farming trick below. I never ran out of water and food without keeping an eye on it too much. You can only drink two coconuts in a row before you're sick so never drink a third, if you wait a few in game hours you can have another two. Break down your empty coconut shell and you can eat the husk, again, you can only have two before you're sick. To increase your health you need reasonably full water and food bars and to not be poisoned. Keep an eye out for Lionfish and Crown of Thorns starfish, if to run into them you're automatically poisoned. The UV index isn't too much of a pain to keep in check usually 2-4pm is the worst, it gives you a warning if it's low so jump in the sea or stand in the shade for a few moments.
Keep your yellow dingy with you for the whole game, it is almost a necessity for the bosses. Keep it with you on your main base. I personally had my main base right next to the 'A Way Out' island (This is an aircraft carrier) for bearing reasons. I personally made my own raft that I used for pretty much everything except for the boss fights. If you use the dingy for a boss fight you can anchor in the middle of the ocean without it drifting away but unfortunately with a self made raft can't anchor in the middle of the ocean meaning you'll almost certainly lose it during the battle.
The technique for all 3 are largely the same. Crafting crude and refined spears and throwing them at the monsters. The crude spears only require one stick to crafted. The refined spears require, one crude spear, one leather and one stone tool. Leather is very hard to come by so I found the following method will help farm leather (And any other materials).
Pick any island that you want to farm on. Back out to the main menu > Cartographer > New Island > Animals Tab > Toggle to the wild boar setting to 2/2. Apply the settings, back out and save, chose any name and any description and once done go to World Editor, and 'Replace Island' on the island you want to replace, back out and save. What you'll find is that your custom island is now where you put it on your map. Go to the island, collect the materials you need, go back to your base, save and quit. Go back to 'World Editor' click on the farming island once again and click replace island > edit island, once again, on the animals tab press apply and save the map. You'll find the boars have respawned along with any other materials you use. As a tip, you can do this for any farming whether it be ships to look for loot, clay, rocks etc.
The sea monsters are in the middle of nowhere, there is no island but one you get close to their area something will spawn. Hook up your sea anchor on your yellow dingy before the fight and initiate the boss fights by either getting close to them or attacking them. Make sure you're accurate with your shots, especially on the squid. Keep a track of the monsters in the fight at all times. *Quick tip, take a sleeping bag with you just before the boss fight and save, it'll save tonnes of time incase you die*.
Crude Spear damage inflicted - 20HP
Refined Spear damage inflicted - 40HP
First up, the Megaladon - HP 500 - Found by a wreckage and a whale.
For this you don't actually need refined spears, I personally used an inventory full of crude spears, I also took 4 bandages in the last slot incase of bleeding. The trick with this shark is to stay close to the surface as possible. Right under the wreckage it seemed there was a sweet spot where it couldn't drag you away. Just keep an eye on your air though. I wouldn't try to stay out of the water as the shark with jump at you and is pretty effective. It will take 25 crude spears to kill. It may take a couple of tries but with a little persistence it's not too hard.
Once killed you'll earn an Aircraft Rudder
The Great Abaia - HP 750 - Found by a Boat underwater.
To start this one, dive to the bottom of the ocean and a health bar with come up indicating the fight has started. Immediately go to the stern of the ship. Essentially what you want to do is hide round corners and throw spears when you have a good enough aim, use the boat as cover. The Eel is quick, it's attacks just drag you away and can cause bleeding, try to get back to the stern as quick as possible to carry on hiding and throwing. Again, with a little persistence it's not too difficult. For this boss I had 12 refined spears, 4 bandages and the rest crude spears.
One killed you'll earn an Aircraft Engine
Lusca the Great - HP 1000 - Found chilling on an anchor.
First, you'll want to anchor your yellow dingy directly next to the metal structure. You'll find the squid down on the bottom of the anchor, be cheeky and get a couple of spears thrown while it's down there. As soon as you've hurt it get straight out of the water, you really want to get out fast as the squid is super deadly. The idea is the get onto the metal structure at the top by getting on your dingy first and jumping over to it. You'll want to throw spears directly at it when you're on the metal structure, as it skims across the top, that's your opportunity . As soon as you've damaged it, move either left or right as one of its tentacles will try to knock you off back into the water, if you do fall in, get straight back on your dingy and back onto the metal structure as quick as possible. It's just a case of spear thrown, dodge, spear thrown, dodge. This will probably take a few goes but isn't too bad if you've saved before the fight. For this I used a full inventory of refined spears and four bandages, after the fight I was left with only two spears so make every one count.
You'll finally unlock your last plane part - Aircraft Propeller
You'll need to craft these parts but they don't require any tools or supplies.
Fix the Plane.
You'll need to do 6 things -
Replace the Propeller
Replace the Engine
Replace the Rudder
Bring some fuel
Bring some water
Bring some food.
You'll need to put the 3 plane parts into the plane which is on the aircraft carrier (A Way Out Island), it's on the top deck (You'll have to jump up the slope going up to the top and follow the stairs up to reach it). The last things you'll need are some food, water and fuel.
I personally used two clay flasks full of water, 12 cooked small meats and 2 full jerrycans of fuel.
The water and meat are easy enough to acquire and shouldn't need directing here. The fuel is a little harder. For this I'd advise using the method for farming above and under the 'Trees' tab you have an option for Potatoes. Click 3/3 and farm these, you'll need 8 in total. Alternatively you can plant potato gardens but it takes for longer to grow than spawning them. Next you'll also need to craft a fuel still, this requires - 1 jerrycan, 2 lashings, 4 sticks, 2 planks, 2 clay and a fire pit. To find the jerry can you'll need to search shipwrecks. They can be hidden in the consoles, lockers or in crates. Unfortunately all consoles, lockers and crates are completely random when you open them, the chances of finding them item you need are abut 5% so just keep searching until you find two, one to build the fuel still and one to transport to fuel to the plane. Again, you can use the 'Shipwreck' tab to farm this if you run out of wrecks. To use the fuel still put 4 potatoes in the top, heat up the still, wait a few in game hours and decanter the fuel into the other jerrycan. You'll need to make two trips for the fuel.
Once you've done that go to the plane and add all the parts, food, water and fuel, once done enter the plane and you'll go though a scene where you follow the commands on-screen. You'll also be treated to a nice ending where you unlock your well earned achievement.