Riptide Turtle

Creature — Turtle (0/5)

Flash
Defender

River Herald Scout

Creature — Merfolk Scout (1/2)

When River Herald Scout enters the battlefield, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)

S.N.E.A.K. Dispatcher

Creature — Human Spy (2/1)

, : Look at the top card of target player's library. If it has an Agents of S.N.E.A.K. watermark, you may reveal it and put it into your hand. Otherwise, put it on the top or bottom of its owner's library.

Sage's Row Savant

Creature — Vedalken Wizard (2/1)

When Sage's Row Savant enters the battlefield, scry 2.

Saiba Cryptomancer

Creature — Moonfolk Ninja (0/1)

Flash
Backup 1 (When this creature enters the battlefield, put a +1/+1 counter on target creature. If that's another creature, it gains the following ability until end of turn.)
Hexproof

Sapphire Leech

Creature — Leech (2/2)

Flying
Blue spells you cast cost more to cast.

School of Piranha

Creature — Fish (3/3)

At the beginning of your upkeep, sacrifice School of Piranha unless you pay .

Sea Eagle

Creature — Bird (1/1)

Flying

Sea Gate Stormcaller

Creature — Human Wizard (2/1)

Kicker
When Sea Gate Stormcaller enters the battlefield, copy the next instant or sorcery spell with mana value 2 or less you cast this turn when you cast it. If Sea Gate Stormcaller was kicked, copy that spell twice instead. You may choose new targets for the copies.

Sea Scryer

Creature — Merfolk Wizard (1/1)

: Add .
, : Add .

Sea Sprite

Creature — Faerie (1/1)

Flying, protection from red

Seacoast Drake

Creature — Drake (1/3)

Flying

Seagraf Skaab

Creature — Zombie (1/3)

Sejiri Merfolk

Creature — Merfolk Soldier (2/1)

As long as you control a Plains, Sejiri Merfolk has first strike and lifelink. (Damage dealt by a creature with lifelink also causes its controller to gain that much life.)

Shacklegeist

Creature — Spirit (2/2)

Flying
Shacklegeist can block only creatures with flying.
Tap two untapped Spirits you control: Tap target creature you don't control.

Shaper Apprentice

Creature — Merfolk Wizard (2/1)

Shaper Apprentice has flying as long as you control another Merfolk.

Shipwreck Sentry

Creature — Human Pirate (3/3)

Defender
As long as an artifact entered the battlefield under your control this turn, Shipwreck Sentry can attack as though it didn't have defender.

Sigiled Starfish

Creature — Starfish (0/3)

: Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Silvergill Douser

Creature — Merfolk Wizard (1/1)

: Target creature gets -X/-0 until end of turn, where X is the number of Merfolk and/or Faeries you control.

Skyclave Aerialist // Skyclave Invader

Creature — Merfolk Scout // Creature — Phyrexian Merfolk Sco (2/1)

Flying
: Transform Skyclave Aerialist. Activate only as a sorcery. ( can be paid with either or 2 life.)

Skyclave Invader

Flying
When this creature transforms into Skyclave Invader, look at the top card of your library. If it's a land card, you may put it onto the battlefield. If you don't put the card onto the battlefield, put it into your hand.