I’ve always found the old maps with imagined sea monsters in unexplored places particularly fascinating. Marina Warner has a long essay in the New York Review of Books about them. The ocean is so interesting because it’s so close yet so impossible to explore, especially before advancements in technology during the 20th century allowed for underwater exploration. The occasional creatures the ocean spits up onto the shore, the glimpses of whales, the sheer terror of the ocean, all of these things contribute to an amazing mythology and power around what covers 70% of the globe.
Personally, I think the greatest value such sea monsters have today is to describe the northern wasteland known in mythology as “Canada” to Americans.