Based on (I didn't see a different way to reference what decks this deck is based on, so I put it in the primer in the hopes you'll read there):

Drown in the Deep | by @ZenatheShadow (Starting point)

Water Control | by @Cthulhu (Additional ideas)

I'll write choices I made based on the differences between these decks.

Added:

• Tufted Turtles - I found this to be a great defensive tool against early aggressions. Also it works well together with Pnakotic Manuscript. I like drawing this in my opening hand, hence why I'm running 4.
• A third Mariner's Curse - Currently testing, I think I want to be able to draw one early on, and I'm not running Mermaids.
• A fourth Riptide - Controlling the board is so important with this deck, and being able to punish the opponent regularly for being close to the body of water is very helpful.
• Bog - While there are already a few defensive sites with Troll Bridge and Floodplain, Bog provides another way to buy a turn if played defensively. It can be awkward very early on, but I like the trade-off.
• UPDATED: Running all four Cores - While I don't want to open 2 or more in my opening hand necessarily, it is still better to run all 4 to have a chance to open at least 1. It allows me to skip a setup turn that I otherwise would have, and allows me to play with low site counts in cases where I am required to funnel an opponent through a defensive site. Any additional drawn only speeds up plays during the late game.
• UPDATED: Philosopher's Stone, Key to the City, Mix Aqua - Similar topic as above, but updated for 50, then 60 cards.
• UPDATED: Silver Bullet - Before I was playing one of the options, Font of Life, at 3, as I sometimes wanted to draw it late. But generally there were more possibilities that I sometimes swapped between. Silver Bullet now solves this issue. Font of Life is now in the collection, just like Ferryman's Coin against Graveyard heavy decks, Marine Voyage to potentially close a game faster, Flood specifically if someone runs Drought against this deck, and Stormy Seas as an option to punish token decks that position themselves too greedily. Marine Voyage and Stormy Seas can be played anytime, while Font of Life, Ferryman's Coin, and Flood are more specific. Flood in addition can also be used aggressively.
• UPDATED: Aaj-kegon Ghost Crabs, Nommo Monitor, Lacuna Entity - Three versions of Voidwalk that also have Submerge. They have good stats, and allow to play both defensively and aggressively depending on matchup. That they can safely move to submerged from the void and back is very strong to avoid getting trapped under a site that might get destroyed. Also gives additional places for Swap to interact.
• UPDATED: Wave of Eviction - It is another way to Riptide opposing enemies, meaning it can interact with minions, can push back even aggressive Avatars like Battlemage, while allowing to force reposition multiple units at once, similar to what Snowball did. Waveshaper then can trap minions in one site, which can buy an entire turn.
• UPDATED: Swap - Extremely versatile interaction, can steal artifacts, drown / void minions, and sneak attack in the back row.
• UPDATED: The Flood - Mass interaction, also another way to permanently flood the entire realm, which is a great way to close a game.
• UPDATED: Croaking Swamp - Good early spending of 1 mana if I don't have usage for that.
• UPDATED: Elder Ruins - Good late game spending of lots of mana. This will need to prove itself, as Island Leviathan still performed really well as a potential threat and closer. This will be a test, so I'll see if Elder Ruins performs better.
• UPDATED: Gilman House - Upgrading your minions / interact with opponents minions.
• UPDATED: Innsmouth Dock - Another site to force move something.
• UPDATED: Mudslide - Forces opponent to consider site placement along the void, as one Mudslide can throw a column into the void.
• UPDATED: Peculiar Port - With Cores / Stone / Key / Mix can this also summon as early as turn 2.

Removed (I'll only list cards that are removed based on that I didn't feel like they were helpful, not based on if I don't have them yet):

• Deep-Sea Mermaids - The opening hand putting cards at the bottom, as well as the Rivers, makes drawing with this card awkward. While it can mean that you setup the toolbox with higher costed cards for later, it also depends on if you draw into Mermaids, and that you didn't have to bottom deck a card for a different reason.
• Grim Guisarme - While it can also make my opponent get to Death's Door faster, it is less flexible compared to Helpful Hob, can make it easier to deal with your own big threats, and it is just as much of a winmoar.
• Abundance - I can see the potential with turn 5 free mana > turn 6 double mana, I think I would rarely ever draw into it, and so it is very much a winmoar card.
• UPDATED: Helpful Hob - Turned out to be a winmoar card, it was mostly great when I was already ahead, meaning I didn't want to draw it when I wasn't.
• UPDATED: Angel's Egg - I know about defensive sites and how to position / defend much better now, which makes these Eggs a winmoar.
• UPDATED: Pollimorph - While strong, I think Wave of Eviction will perform better in Waveshaper.
• UPDATED: Rivers, Pond, Tadpole Pool, Maelström - These are removed to make space for the new sites. Pond was not bad, but Peculiar Port should perform similarly in summoning something early. Same as Croaking Swamp. Tadpole Pool rarely came up during playing, but it is on the maybe. Maelstrom was usually too awkward to deal with and never came up.

Considering:

• Shrink - Currently thinking through the possibilities, as it could help dealing with early aggressive threats.