I could not help but constantly think of "Dagon" while I read this story. I think this was a very interesting lesson in why "Dagon" succeeded, where "The Leviathan" failed. "Dagon's" strength lay in...
1. A fantastic, grotesque monster. This one had that too, so well done.
2. Accurate description of a sanity shattered narrator. This one did okay, but I could not even CONCEIVE of the suicidal narrator of "Dagon" even daring to LOOK at the ocean, much less cross it back to America.
3. Horrible implications beyond a mere "monster of the week." That is "The Leviathan's" biggest failing. It's just a monster. Sure, it's big - but why would that shatter your sanity? Lots of things are big. In "Dagon" an entire culture of unspeakable creatures exist, worshipping such eldritch horrors as gods. In the end, the narrator may have actually been pursued by those fishy horrors even on land...
The writing was okay, but beyond the monster, it did nothing. "Dagon" (and even moreso "The Call of Cthulhu") are shining masterpieces of this sub-sub-genre and everything will inevitably be stacked up against them. I think this piece failed to be do anything different or new than other titans of the genre and likewise failed to even achieve a decent pastiche or imitation of Lovecraft or others.
Even worse, it seems our indomitable host Alasdair was deceived: he makes reference to how this story conjures up horrible implications of terrible, unseen worlds lurking just beneath the waves - so very close to home. But he's wrong, this story conjures up memories of OTHER stories which imply those things, this story just has a big scary monster in it with none of the unspeakable implications of MORE horrors to come which made Lovecraft supreme master of the sea monster. Seeing Lovecraft's monsters was more than just seeing a monster: it proved that all the nightmares you read about in that forbidden book are true! It implied unspeakable things were to follow! Dagon, Cthulhu - they were just the barest TIP of the iceberg. Mere heralds of even worse nightmares to follow - and WORSE after that! All of it unavoidable! We will all perish! The Old Ones will rise again when the stars are right and mankind will be at an end! Ia! Ia! Cthulhu fhtagn! Ia R'lyeh! AHHHHHHHHHHHHHH!!!
The Leviathan? It's just a big snake.