any thought to Triton Shorestalker ?

None at all. I already have 8x 1cmc permanents, no need for 12

so i absolutely love your deck. I am changing my merfolk to include cosi, however there are a few things i would recommend;

1. fetches?????
2. cage - if pod was still around im all for it, but now that its gone not so usefull
3. phantasmal image - when would you bring him out of the sideboard
4. cavern of souls - STRONGLY recommend 4 caverns especially if you put kira sb so your deck isnt as nonmerfolk blue mana dependant
5. swap tidebinder for threads of disloyalty. only things tidebinder really does for us is tap goyf or hatebears. why not grab it for extra damage.
6. the ongoing war - Remand vs. Spell Pierce. this deck likes spell pierce more. reasoning - you dont care if they get a creature, you dont like combo parts going to their hand, and if they can pay the 3 mana, youve probably lost anyways. the upside you lose is that extra draw, but when you have 8 other draws MB not a heavy loss.

these are just my opinions, but i hope they help.

Well I don't know what you mean by fetches??? Because I'm running 4 and I love them. It helps thin the deck so well. So I'm effectively running 14 lands that want to play like 20.

Grafdiggers cage is a meta call people are starting to brew with GGT and I still see tons of Vengevine being played as well as Bloodghast. Also, living end is a deck that I just want to shut down.

Phantasmal Image was primarily there for super aggro decks that I need to race. It actually does some work helping race those decks. Moar Lords!!!

I'm not moving up to 4 caverns, mainly because I haven't updated this to my most recent build. Mainly because there is no counter control in my meta so I don't need it. But secondly I moved out the Master of Waves for Kira. That had multiple reasons but primarily because by the time Master is relevant to cast I've either won the game or lost the game. And it's just not smart to try and Vial him in.

Threads is a solid card. I tested it. But honestly the only deck in my meta that has Goyfs is TarmoDelver and guess what? I don't care about his Goyf when I just swing right by it. But again, I think it's more relevant to have a dude who can tap down scooze, Goyf, Jund Dragons if they come back, Vengevine, Goblins for days, AAAND it gets to attack. Seems good. And probably can't be countered. Unlike threads.

Spell pierce I was going to run but you know what, since a lot of times spreading seas gets sided out I find it all to relevant to play Remand instead. So I can take out two seas, put in the remands, have my counter control while still cantrip for days. Also, spell pierce says pay two. And there are creatures I want to counter occasionally. And combo decks know how to play around the two mana for a pierce. So who needs it? I'm not trying to get into a counter war, I just want to hold off a combo piece or an Anger for a turn.

That was probably way more info than you needed or wanted but yeah. Tonight I took first place again at my LGS in a 10 man tourney doing four rounds top 4 and never even side boarded winning each match in two. Granted, I played some janky decks, but still. Game one I beat UW Heroic, game two I beat up UW Allies which tried to mill and life gain me to death. Failed badly. Game three was America Twin which I stomped without having to blink, and game 4 was TarmoDelver and again TarmoDelver in top two.

Anywho. I'm glad you liked the deck! The side board will probably change drastically when It actually comes time for my next GP which is GP OKC this fall.

Hit me back with your thoughts!

Grafdigger's Cage actually doesn't work against Living End decks. I had to learn this the hard way, unfortunately.

Oh yeah I can see that now since Living End exiles the creatures first then they enter from the Exile Zone. Which sucks. Well if its really that relevant then I guess I just don't really need a 15 card SB. Because there is literally nothing else this deck wants right now.

Unless someone can think of something?

If Living End is a common deck in your meta, all you really need to do is replace Grafdigger's Cage with some Negate. While you can't counter the Cascade trigger itself, you can most certainly counter Living End while it's being cast on a Cascade. Bring in your Remands and Negates, and save them just for Living End, and the deck can't beat you.

It's not common at all, I just want to have a sideboard ready for all major deck, and now that Pod is out that really negates half the need for Negate and most other counter magic. Really only thing I need to be prepped for is the random red decks that have Pyroclasm or Anger. This deck has placed in first or split for top two every week since I brought it out of retirement so I really see no reason to make changes.

For me Relic of Progenitus has taken the place of Grafdigger's Cage. It slowly eats away at graveyards to hold off Tarmogoyf, Snapcaster Mage, and Tasigur, the Golden Fang as well as being able to mass exile everything as a panic button. At its VERY worst it's at least a cantrip too.