Cursecatcher
Allows for a Turn 1 creature that can attack on Turn 2 for 2 damage if a lord is played on Turn 2. May not be as good as a Spell Pierce in terms of countering ability, but the ability to do so and being on a body is more than worth it. I lose out on the ability to Aether Vial it in due to budget, loses unpredictability, but still needed due to being a 1-drop. Creates potential for Turn 4 win (Play Turn 1 Cursecatcher, Turn 2 lord and swing for 2 [18], Turn 3 2 mana lord and Sea's Claim, swing in for 6 [12], Turn 4 lord swing in for 12 [0]). Also helps the mana curve.
Merfolk Trickster
A 2/2 body that can swing in for 3 or 4 the following turn that it's summoned. Most often used with its Flash ability. Tap creatures such as bombs or if they have abilities that work against you for tempo. Having 2 untapped Islands also add to unpredictability, could be Spell Pierce, Negate, etc. 3 of to make room for everything, as well as being the most flexible spot due to having more utility Merfolk, such as Cursecatcher and Harbinger of the Tides.
Harbinger of the Tides
A 2/2 body that can swing in for 3 or 4 the following turn it's summoned, similar to Merfolk Trickster. Can be flashed in for 2UU, but ability is unusable due to loss of tempo. Also cannot use Aether Vial. Can use after opponent attacks to bounce back to their hand and will have summoning sickness on following turn, resulting in gained tempo. 3 of for the same reasons as Merfolk Trickster.
Lord of Atlantis
The first Merfolk lord. Play after Silvergill Adept for added tempo, but save Merfolk Trickster and Harbinger of the Tides depending on match-up. Sideboard out against Merfolk ditto due to giving opponent's Merfolk a buff. Gives Merfolk Islandwalk, combos with Sea's Claim.
Master of the Pearl Trident
Similar to Lord of Atlantis, play after Silvergill Adept, Harbinger of the Tides and Merfolk Trickster optional. DON'T sideboard out in Merfolk ditto. Gives Merfolk Islandwalk, combos with Sea's Claim.
Merrow Reejerey
3 drop lord. Allows for untap when casting Merfolk spells, doesn't have to resolve. Untap lands turn 4 to empty hand of Merfolk. Also taps potential blockers or threats. Combos with Merfolk Trickster to tap 2 creatures on opponent's turn.
Master of Waves
Major bomb, creates many 2/1 Elemental tokens. Bringing out more than one creates even more, and lord ability for Elementals stacks. Protection from Red gives advantage against Burn.
Phantasmal Image
Creates 2 mana copy of anything needed. Still mimics enter the battlefield abilities. 1-of to make sure variance is still as low as possible. Increases variance.
