The story is pretty heavy in Subnautica: Below Zero, and as you progress, you will have to search for your sisters work site, and complete her work.
To find the Frozen Leviathan differs a fraction depending on the location that your Drop Pod landing, but the Frozen Leviathan is always in the same location—in the Glacial Basin.
The Seatruck needs to be fabricated along with the Seatruck Depth Upgrade MK1 and a Cold Suit to equip before setting off on your journey.
From your starting biome Twisty Bridges or the Drop Pod, her location is northwest of the Glacial Basin, in the Phi Excavation Site. From our starting location/drop pod was go southwest about 1000+ meters until you start seeing Eye Jellies.
You'll see a massive Ice Wall/Structure in this area. You need to swim to the ocean floor about 120m+ depth and find a way underneath it. If you are in the right location, plenty of Eye Jellies are grouped, so be careful not to hit them.
Underneath this structure, there's a cave you must enter, but it's easy to get lost within it, and of course, Eye Jellies are still floating about. When in the cave, keep to the section that is elevated. If a path is going further down, do not take it. Eventually, they'll be an opening above you that you can take.
When on Icey ground again, look northwest, and there will be a cave entrance with glow lights outside it. Enter it, and keep to the left to find the exit to an area called Glacial Forest, full of purple trees. From the cave exit. Head northwest until you see a radio communications tower in front of you.
To the left of this tower is another cave. Go through it, follow the path and you will find the Frozen Leviathan.
If you wanted to find it without exploring, here are the coordinates for the location -1619 29 -763. Use the Console Command- warp -1619 29 -763