WE ALL SERVE THE FISH.
Reject your opponents' spells. Keep milling their deck. Keep looking for the fish. When the fish emerges, it's time to shine.
REJECTION
We have Disallow, Essence Scatter, Censor, Winds of Rebuke,
Nimble Obstructionist
, and Baral's Expertise to deny all those spells that disturb the fish. Supreme Will provides utility for both countering and the search for the fish, and Siren Stormtamer is pretty good against red and black.
MILLING
The fish may not be drawn every time. Therefore, we have other mill cards that work great, especially with Fraying Sanity. Winds of Rebuke mills four. Seer of the Last Tomorrow mills six per activation. Ipnu Rivulet and Navigator's Ruin eight per go. Compelling Argument ten.Minister of Inquiriestwelve in total.
SUPPORT
Oracle's Vault is very useful in your search for the fish and with a bit of luck, you may even be able to cast it for free!
The sideboard cards
Spell Swindle
and Thaumatic Compass are great at dealing with high-cost hard-hitters, with one countering high-cost cards for mana reward and the other being able to remove creatures from combat.
Overflowing Insight
can be great when looking for the fish, but 4UUU is a lot. The reward is high, though, so it is considerable.
I know that Imprisoned in the Moon and Summary Dismissal are not standard legal anymore, but i just included them as they're budget and can be useful in casual matches.
THE FISH
The fish, a.k.a.Fleet Swallower plus a Fraying Sanity is game over. The fish mills the top half, Fraying Sanity finishes the rest. Clean and Smooth.
THE FISH SERVES US ALL.
The fish does serve us all, but it is still okay if we don't get it. With this deck, the best case scenario without would be:
Turn 1 - Island; Minister of Inquiries. Opponent has 53 cards in deck.
Turn 2 - Island; 2 more Minister of Inquiries. Opponent has 52 cards in deck.
Turn 3 - Ipnu Rivulet; Fraying Sanity; tap all 3 Minister of Inquiries; mill 18 in total at the end of turn. Opponent has 33 cards in deck.
Turn 4 - Ipnu Rivulet; Compelling Argument (tap 1 Ipnu Rivulet to cast); sacrifice the tapped Ipnu Rivulet with the other Ipnu Rivulet; tap the 3 Minister of Inquiries again, mill 36 in total. Opponent has -3 (0) cards in deck.
The game is yours.
this scenario is completely hypothetical and your opponent could actually get milled out quicker due to several reasons (self mill, discard/cycle, etc.). He/she could also have countered your mill spells or had effects to puck cards back into the deck (Commit // Memory, for example, of they ramp quick enough). Also, this is only if I didn't have to play any counterspells to avoid getting killed very early, which would slow me down.
This still shows how this deck can be very deadly even without usage of the fish.