General strategy: Back yourself into a corner, keeping Free City and Bottomless Pit on the front line, keep Mariner's Curse on your only water sites, and play out your 4 sites, drawing into sites if needed to get to your defensive utility sites, and stop at 5 ideally before using your ability each turn to keep yourself safe. Pendulum of Peril will require drawing an additional site. But until then you can use Shackles / Drown / Pollimorph / Stormy Seas to deal with enemy minions that manage to sneak past your defenses, and ideally you will eventually get down your abundance and/or Throne to take over the game.

Mulligan hard for:

Spells: Mask of Mayhem / Angel's Egg / Immortal Throne / Mariner's Curse (MAYBE Riptide, it's often the best card in the deck but you're also very likely to see a couple without digging for it.)

Atlas: Free City / Bottomless Pit / Undertow (If you don't have a Throne AND don't have any utility lands, 1x River may be useful to dig for what you need.)

Mulligan out of:

Spells: Pendulum of Peril, Hydra, Wrath of the Sea, etc. Stick to 2-3-4 mana spells.

Keys to Victory: Use Mask of Mayhem + Free Cities + Bottomless pits to force bad attacks on our enemies so we can continue to tap for sites as long as possible. With Free City doing 6 damage this should kill most minions. Mariner's Curses shore up this plan to protect our regular old water sites, and a well placed Blizzard can either prevent your death or save some life to get to our eggs, or the sweet combo of mask + blizzard means they must attack what we want them to (force an attack into a suicide site.) We really want to win with the Immortal Throne, but can finish the game with our little minions if we maintain control, or a couple hits from Seirawan Hydra, Angler, or eventual Island Leviathan.

General Reminders: Don't blow your Riptide to move your avatar and draw a card, in this deck it's often a hard removal spell or tons of avatar damage. Undertow is also massively powerful but you need to set it up by flooding a Free City or Bottomless pit *before* playing your undertow.

Know when to hold your Mixes, or 1 drops in general. This will pay off if you need to play Throne, site, 1 drop, but also as a good habit for Roots decks. Top decking an abundance on turn 2 that you COULD play if you had just put down the mix the previous turn feels horrible.

Cast your Accursed Albatross directly onto your Bottomless Pit usually, but can also protect your avatar.

Porcupine Pufferfish usually sits under your Avatar for defense.

Playing rolling boulder for the 4 drop for throne can be a trap, as it's really there as a finisher when we dont get the throne. Proceed with caution if your health is low, using it too early, or if the opponent has the mobility to teleport/blink to the boulder and use it against you.

Cards I am iffy about:

Albatross can be damn good, it can also be too expensive and small and do very little against decks like Battlemage or Sparkmage or anyone running few minions.

Abundance is a lot of mana on turn 5 when we should already be tapping to control their army.

Drown (just removed, still experimenting) and Iron Shackles are not always premium removal, either could be something else.

Pollimorph is decent, but not amazing. Have obviously considered Sleep and Shrink and other removal options as well.