Call me Ahab
Unfortunately with this achievement I can only give you tips and a bit of guidance. The location of every sea monster 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)
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 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 - 40HPFirst 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.The Great Abaia - HP 750 - Found by a Boat underwater.
To start this one, drive 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.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.
Once you've killed each boss you'll unlock two things, an aircraft part and a trophy. Craft all three trophies to receive your achievement.