This is my take on an aggressive Islandwalk deck made to deal with as many deck types as quickly as possible. The goal is to get the Merfolk Lords out and start making big threats as early as possible.
Key Pieces
Aether Vial - The majority of creatures in this deck are two drops which the vial suits perfectly. Able to play a creature at instant speed without worrying about counterspells makes this suitable for combat tricks and ETB effects that may not be needed on your turn.
Lord of Atlantis & Master of the Pearl Trident - There's no real need to separate these two. Same casting cost, same power and toughness, same effects. It's almost like having eight in the same deck.
Spreading Seas - As much disruption as it is an enabler, being able to replace itself only makes this card better. Being able to early on turn off an opponent's land mana by making it either only one of their colours or none of them can buy a couple of turns. The addition of making it an Island can easily turn opponents on to a three or four turn clock depending on the number of creatures and lords already out.
Other Creatures
Cosi's Trickster - In a format that is filled with Fetch Lands and tutor effects, this is one of the creatures that can become a big threat if not removed early on.
Harbinger of the Tides - Bounce on a stick is great. The versatility of it is good too. Either play for two, four with flash or simply vial it in.
Kira, Great Glass-Spinner - Can easily be a big pain for decks that don't run cards like Abrupt Decay or Combust since they are wasting two spells to remove this or other creatures.
Merrow Reejerey - Initially I ran this but switched to Merfolk Sovereign when I found that the alters I'd made didn't really benefit the tap or untap ability of the Reejerey but the unblockable ability on the Sovereign would help when having trouble activating Islandwalk. But I've found that now running less blue mana sources makes the Sovereign harder to cast and the addition of Harbinger of the Tides gives additional synergy with the cast to tap ability on Reejerey that makes it the better option in this build.
Silvergill Adept - Two mana for a 2/1 body with draw a card is pretty good. Being able to vial it in without paying an additional three or revealing a card from hand is also pretty good.
Sygg, River Cutthroat - Having repeatable card draw is very good, especially when that draw is made from an opponent losing three or more life each turn.
Instants
Remand - Disruption is key in tempo. Replacing itself whilst sending back your opponent's spell to their hand, letting you keep up something for it in the next turn, is very good to help keep that tempo going.
Rune Snag - Unpopular opinion time, I prefer this to Mana Leak in this deck. Tempo is key here and being able to make a counterspell that asks for a mana payment still be a threat by turn six or seven is much more favourable than one that asks for three mana and only work early game.
Vapor Snag - Instant speed creature bounce for one mana is a good but the fact that it targets both sides makes it either a good saving throw against some removal or a combat trick that either prevents lethal by bouncing an opponent's attacker or a way of giving lethal by getting that one extra life from them.
Non-basic Lands
Cavern of Souls - Mostly a measure against counterspells on creatures that can't be vialed in. It helps make early and late game board presence easy to do.
Ghost Quarter - Sometimes you just need land destruction. Compared to Tectonic Edge, I believe this is the one I'd rather use for Merfolk. I would rather hold up that one extra mana for a counterspell or use it to activate Mutavault for blocks or attacks. Plus the lands I would rather do it to (Urza lands, Eye of Ugin, Eldrazi Temple) are ones I would rather do it to early on so they can't ramp quicker than I can build board presence. Waiting to turn four would quite likely kill me.
Mutavault - A manland that becomes a Merfolk that dodges sorcery speed removal is very useful for turning extra mana into attackers and blockers.
Sideboard
Dismember - It's a given that this deck has a hard time permanently dealing with creatures due to being mono-blue. So paying four life and one mana is fine for taking down the big beasties.
Echoing Truth - Mostly token creature removal. It's also a good bounce spell for decks that run multiple copies of permanents that can lock down boards easily.
Grimoire Thief - An extra bit of counter that helps to disrupt combo decks and mill out important cards. The fun part is that it can actually make an unobservant player draw themselves out if they don't take it into account how many cards they have left in their deck.
Gut Shot - Infect, Abzan, Elves and Zoo matchups are where this gets sided in. If a creature like Glistener Elf or Birds of Paradise needs to be removed early on, this can do that on Turn 1 when they won't be able to respond.
Hurkyl's Recall - Mostly a counter measure to Affinity and 8-Rack. It can also be used to save Vials though that is really not used much.
Monastery Siege - Mostly for use in the Dragons mode though occasionally the Khans mode is activated if a deck with low removal or targeting is being played against but also draws faster than us like Affinity or Elves. Having to pay {2} each time to cast a Path to Exile or Lightning Bolt is not good value.
Tidebinder Mage - Zoo, Elves and Jund Creatures are fairly hard to deal with at times and can be two of the few creature heavy builds that out speed Merfolk with dealing heavy damage so being able to tap down heavy hitters like Tarmogoyf and Wild Nacatl until it is removed helps a lot.
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