This is a well thought out suggestion with many details! I like everything except for the mobs, they just don't fit in with the other mobs, and with almost no other sea life, they are basically the only things there. Also, they're too big. The ender dragon should be the only survival-friendly huge mob, in my opinion. Mostly Supported.
Going along with the mythical creatures... there could also be some more sea creatures, like some types of sharks and stingrays (that only attack when provoked) and some others... which drop custom items... which would be a lot to think about.. but Yea, Awesome Ideas! Hopefully it's added... SOMETIME... since this was posted over 2 years ago... but then again.. looking forward to it!
I love the concept and the vision, it's really cool! The mod still feels a little empty though -- you have your centerpieces, boats and gigantic monsters which threaten the boats, and some tools with which to better explore the ocean... But what else? The ocean would still be somewhat empty -- the two main things this concept needs in order to be fleshed out enough; firstly, it needs more regular ocean life (both to liven up the ocean, and also to give the Leviathan a sensible origin), and incentive to actually take to the ocean in those vessels and risk being attacked by the monsters. The Kraken is a cephalopod, and there are already squid, so you're good there -- but what about the Leviathan? A mutant fish? An overgrown eel? The Leviathan needs to be expanded on, I think. And then, as for incentive to head out in your ships... See, I don't think just travelling across the ocean to find more land is a good enough reason -- players can do that on land, and there's much more to see on land. In fact, speaking of land... Maybe there's a biome that can only spawn in the center of a large Ocean biome, a colossal island, which the player might seek to find and explore? Or maybe, under the centre of the ocean, there's a massive cavern filled with exotic sea creatures and strange treasures! Perhaps there's a chance for foggy weather to arise, in which there's a low chance that the players' ship will cross into an 'Edge of the World' dimension! The monsters are supposed to be exciting in a dangerous way... So I think that there should be something exciting in a wondrous way to counter that, and be worth going through them for! That's just my opinion though. I realise this is an old thread, but I think it still has lots of potential!
Also, as an idea for another monster... How about a Moby ****/Monstro type? Man the harpoons! xD
Naturally, I think that would call for normal whales to be added too... Which would be another good way to put some life in the sea! In addition, there are lots of things whales could be good for (such as whale blubber, LOTS of meat, sheer wonder, an excuse to add krill, etc), making them another good potential incentive for players to go seafaring!
Come to think of it, another appropriate monster could be a Megalodon. Sure, it might not be as big as some of the other monsters, but it's probably hella vicious! Plus, again pertaining to monster origins... Sharks! A popular sea creature that also has its uses, such as its teeth, its fins, its meat, its cartilage, etc.
I'm going to - for now - go with Partial Support, because while I love the idea and where it was going, it's not ready yet.
Potion of under-water-breathingness beats snorkel.
The potion of water breathing does not beat this, due the fact that brewing requires diamond level gear, a trip to the nether, and each potion is temporary. The snorkel is easy to make, and only requires killing a couple of turtles and spiders.
Leviathan is a monster from the bible. It is described as being a mile long, having scales so hard that they will shatter swords, breath that boiles the water in front of it, and a stench of rotten fish so bad that it ruins cities. It was a monster so terrible, that god had to kill the male so they couldn't reproduce. God's also going to kill the female at the end of times, create a tent of it's hide, and have a BBQ with it's meat. Isn't the bible wonderful?