This deck is centralized around Chasm Skulker + Stormtide Leviathan , getting a ton of small squids and striking with them all at once thanks to the mighty power of islandwalk.

To power the Skulker, we're going to need to draw a bunch of cards, hence Arcanis the Omnipotent, Treasure Cruise, and Bident of Thassa. Dictate of Kruphix + Floodtide Serpent form a nice little combo: Flash the dictate at the end of your opponents turn so you get to draw two, then return it to your hand with the serpent on your turn to prevent your opponent from getting the extra card.

We've got several high-cost creatures, and we don't want to spend the mana to play them, so Jalira, Master Polymorphist, Quest for Ula's Temple make sure you can almost always play what you need. Skill Borrower, in addition to having a useful ability, lets us see what our next card is at all times, so that we can make sure a creature is at the top for the beginning of our upkeep. Thassa, God of the Sea is there for similar reasons.

Finally, Evolving Wilds helps to thin out the deck. Elixir of Immortality gives you some survivability, as you will be putting a lot of cards in your graveyard, so shuffling it back into your library can help out at times.