Core Cards:

4x AEther Vial, Cursecatcher, Lord of Atlantis, Master of the Pearl Trident, Mutavault, Silvergill Adept, Spreading SeasHighly Recommended Flex Cards:

Merrow Reejerey - the number played kind of varies, with some decks running four, some decks running one, two, or even none, but I've found three to be best. Helps you curve out to Master of Waves, and sometimes being able to untap lands after casting a fish can win you games.

Master of Waves - A lot of people hate this card, but it's probably my favorite in the deck. It will literally win you games against Jund, and is extremely strong in other matchups as well.

Spell Pierce - Another card that's kind of on the outs with a lot of people, I really like it because it can save you in so many matchups - keeps Liliana off the board, stops that burn spell heading for your face, or can interrupt a combo at the perfect time.

Other Cards:

Kira, Great Glass-Spinner - I bring her in and out every so often. When she's good she's great, when she's bad she's terrible. Really a personal choice, but I'd rather have her in game one than side her in later on.

Harbinger of the Tides/Tidebinder Mage - Personally I'm liking the 2/2 split, but I was running 4 Harbinger main 2 Tidebinder side for a while. At worst they're both 2/2 bodies for 2 that can get boosted by a lord, at best they can slow down or completely stop an opponent's deck.

Non-Island Blue Sources - Primarily for Choke/Boil, but also EXTREMELY beneficial in the mirror as they don't grant island walk. Plus Oboro can bounce back to your hand in response to a discard spell.

Sideboard Thoughts:

Remand - it's won me so many games, stalling out long enough to let me just finish off my opponent. When they tap out for that big spell and can't cast it again, send it back and draw another card and you can probably win next turn.

Gut Shot - This one's probably going to be taken out. It's supposed to be great against affinity, but it hasn't done anything for me yet.

Tectonic Edge - Against grindy matchups like Jund, AEther Vial comes out on the draw and Tec Edge comes in both to help your curve and to take care of those man lands.

Hurkyl's Recall - Yeah, 3-4 is a lot for a card really just used for affinity (and against Ensnaring Bridge if you can get it to your hand before they mill it out of your deck), but honestly that's just how bad the matchup is.

Chalice of the Void - The more popular choice over spellskite these days, I just haven't made the switch to 4 chalice (replacing 2 spellskite and 2 gutshot) yet because of budget, and spellskite has served me well in the meantime anyway. I might actually try a 2/2 split in the side between Chalice and Spellskite to see how that goes.