This first link is an interesting array of small images of various sea abominations.
There are a few illustrations by Warwick Goble of sea monster attacking in this link. I really like these two images and the text accompanying them is great.
"A sudden appearance out of the sea of a race of amphibious monsters, capable of wiping man out of existence."
"Loathsome animals of huge size will creep over the masses of ice."
Here is a quote from the article related to the above illustrations taken from here.
"Fossil remains of crabs, 6ft. in length, have been discovered, and such enormous creatures might -- owing to some cause or other -- multiply exceedingly.
If we imagine a shark that could raid out upon the land, or a tiger that could take refuge in the sea, we should have a fair suggestion of what a terrible monster a large predatory crab might prove. And, so far as zoological science goes, we must, at least, admit that such a creation is an evolutionary possibility.Then there are the cuttlefish, the octopus, and other denizens of the deep, any of which might conceivably grow in numbers, and extinguish man. "
Click the image on this link to see a larger version of this depiction of a whale and other sea monsters.
There is an interesting depiction of a sea serpent here.
This old map of Iceland has some interesting illustrations of fabled monsters around the island.
This link has no artwork but there are some good descriptions of legendary sea monsters.
Last but not least, here's pictures of real sea monsters.