I believe it's big enough it wouldn't have had issue making it to shore from the island if it'd wanted to go onto the land.
It survived sealed into an undersea cave for who can even say how long, without food, sunlight, or perhaps adequate oxygen. Who can even say how ancient this beast is? Perhaps it's a marine creature by nature, but if it didn't head towards the land, I don't imagine it'd submerge itself in an ocean it wasn't certain it could survive. All creatures have some basic instinct towards self-preservation.
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It survived sealed into an undersea cave for who can even say how long, without food, sunlight, or perhaps adequate oxygen. Who can even say how ancient this beast is? Perhaps it's a marine creature by nature, but if it didn't head towards the land, I don't imagine it'd submerge itself in an ocean it wasn't certain it could survive. All creatures have some basic instinct towards self-preservation.