So, just a quick summary of all of the card choices. This is a control deck, meaning you should aim to go first. Your main goal should be to protect your field spell. As long as you can do that, you should be fine. One of the coolest things about Pacifis is the versatility of the deck. Don't be afraid to try different builds.
Monsters
Spiral Dragon: 3 is too bricky, 1-2 should be enough. I've seen several decklists that just don't use him at all, but I think he is necessary to close up games a bit quicker.
Megalomasher X & Mystery Shell Dragon: The goal here is to have 3-6 normal monsters, of course, other builds would have different rations of monsters. I went with Megalomasher b/c he is a dino (fossil dig), and he can summon Bahamut. Mystery Shell Dragon is a wyrm, meaning we can use him and spiral dragon for tenyi link plays. Plus, both of them have 2000 attack, which is the highest a normal monster can have.
Dimension Shifter: A few months ago this was ash blossom. I am not fully sure what the best hand trap would be here, as there are others I want to try, but these slots can go to whatever meta staple fits in your current format.
Spells
Heat Wave: Can shut down almost every deck, without hurting us one bit. Must have at 3.
Fossil Dig: See Megalomasher X
Terraforming: We need this because, despite all of the support Umi has, it does not have a searcher.
Phantasm Equip Spells: 2 of each except for wave, as we need to wait until the end of the battle phase to special summon a spiral dragon. Thankfully, it can make your opponent discard a card, so it isn't all bad.
Pacifis: Your key card, the thing that makes the deck viable. Must have at 3.
Traps
Metaverse: Since terraforming is limited, we have to run this as well.
Phantasm spiral battle & power: Become hand traps when Umi is on the field. Battle is spell speed 2 destruction, while power is an in-archetype infinite impermanence. They also have their second effects, which come up, but are not essential to victory. We don't use trap trick because their appeal is in the fact that you can play them from your hand, and trap trick puts them onto the field. 3 battle and 1 power, but I am considering changing their ratios.
Sea Stealth Attack: Can recycle our Pacifis, protect it and our floodgates, while making our spiral dragon and token undefeatable in battle. 3-of.
Summon Limit: A floodgate that does nearly nothing to us while ruining our opponent's career. Sure, we cant make as big extra deck plays, but its not like we needed it to win.
Gozen Match: Another floodgate, as one can never have too many floodgates. Spiral dragon, ur phantasm tokens and megalomasher X are all water attribute monsters, so this card shouldn't hurt u much. Cant say the same about ur opponent. I am considering bringing it up to 3, but for now its at 2.
Skill Drain: The boss of all floodgates, this card can win you the duel on its own.
For the extra deck, you can do whatever you want, or not run an extra deck at all. It's your choice. Note that other builds, the Extra Deck may have a lot more importance in the duel. In general, the only deck that shouldn't be running an extra deck at all is Monarchs.
The side deck right now is a mess, filled w/ ideas and speculation. As I perfect the build, the side deck will become clearer. For now though, I would recommend at the very least nibiru, droll, and dark ruler no more. If u have better cards in mind, go for what u thought of, I'm not the best deck-builder in the world.
Overall, pretty fun deck to play, maybe not so much for your opponent. But at least it's not mystic mine. All suggestions are welcome and appreciated.
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