True true.

I did some reasearch and theres no other description other than large, monstrous beast, which resides underwater

What about the ice worm

Ice is frozen water

But ice in large amounts is land not sea

Don’t the PDA entries state if they are leviathans?

^

Yes, but the question is why are they leviathans and how could you tell if other creatures are levithans

Glow whales are the smallest leviathan and exactly 30m long. I think it’s simply based on size.

But then Sea Treaders wouldnt count as leviathans

I don’t think the makers really thought that one through, lol.

During Early access of Subnautica, this has been a back n forth debate. Earlier days, the reaper was treated as the smallest requirement on the leviathan chart. However, in-game these definitions and categorizations were changed to be more open to include Sea Treaders and Reefbacks.

Another point of contention is the Squidshark. The PDA classifications for it are a little messy with it being labeled as a "leviathan" but then not under the tab category of "leviathan". Probably a detail overlooked.

And this is a 'leviathan' only 11 meters long.

TLDR:

Pick one or the other, Either subnautica classification of 30 meters as smallest for leviathan, or 11 meters from below zero as smallest for a leviathan.

Yeah There isn't any specific criteria for an organism to be a "Leviathan". I think we can just assume any creature that's large in size and has a large ecological impact can be called a Leviathan, or just go off what the PDA says