**Don't forget other lifeforms**

I think the most heavy living being on earth was a mushroom that does have a mass of over 100 tons... 
Well, while this may not be a leviathan as described in your question, you should take in account that corals are animals too, If you consider a coral reef as a hive-like animal, they may exceed any other animal ever existed in size.

And it is an underwater-animal, so this somehow does apply to you question. Of course, coral reefs can sink ships, but usually they do not tend to be the most aggressive animals in existence (more like the most passive ones). 

But if you are not afraid of handwaving, you can construct a coral reef of the size (and mass) of an aircraft carrier that is kind of intelligent and goes for new sources of food by itself. Add some tentacles and... wait, don't do this.