I won an extended FNM with this (Matchups below). I do not have a solid grasp of the Extended meta since I usually just play Standard, but I was out-of-town and that was the format du jour. This was just an exercise in building a deck with cards on hand and the results are a datapoint. Feel free to discuss.
The deck is basically an amplification of the card-drawing capability inherent of merfolk, and folds in tempo advantage with Standstill and recursion with Lark.
MU 1: Saprolings...
This deck was able to chump block for about 6 turns each game before merfolk charged to victory. I don't think my life total moved from 20 either game. 2-0.
MU 2: Mono-white Equipment...
This deck actually held up for awhile game 2 after it boarded in and produced multiple copies of Shield of Duty and Reason. A 6/6 Shielded creature with Vigilance beat on me for a couple of turns before I could overwhelm this deck with an army of merfolk and incredible card advantage. 2-0
MU 3: Tron
Game 1 the merfolk spotted an Island on the far side of the table and ended the game in 5 turns. Game 2 I boarded in Pact of Negation. Thinking I was well in control of the game with a small but competent merfolk force, I played Pact of Negation to counter Razormane Masticore. Opponent smoothly proceeds to play, and use, Mindslaver, and decides it would be best for me to lose the game the following upkeep. I humbly oblige. Game 3 we are both at 4 life, and Opponent has 2 Razormane Masticores in play with 1 card in hand. On my turn, I evoke out Lark, bringing back 2 Mulldrifters, but I'm tapped out. Had I just played out Lark for normal cost I would have won this game, but instead my opponent loses 1 Masticore, kills a Mulldrifter, draws into O-Ring and swings for the win. Oh well, it wouldn't be the last time we play today... 1-2
MU 4: UB Faeries
Game 1 I don't hold a Lord of Atlantis but decide to keep anyways. Opponent Mulligans to 6, 5, 4, 3, then concedes. Game 2 I do draw 2 Lord of Atlantis, and the game is over almost before it begins since this particular Faerie deck actually has some Islands. 2-0
MU 5: Tron
Same Tron, but this time the games were much easier. First game I led with a Cursecatcher, Standstill, then dropped a Mutavault and forced opponent to bust Standstill to catch up, which they never did. Game 2 was similar, with an early Hoofprints of the Stag followed by a Standstill, which I find both comical and scary. Standstill was really good on this night, and though I rarely drew into Hoofprints I was really glad to have it, since it plays like a gargadon on Steroids in this deck. This time Pact wasn't met by Mindslaver and I rolled to an easy win. 2-0
The deck worked well because even after dropping an early army I had a full hand of cards and lots of options. I probably Pondered ten times and shuffled in all but one case. I had fun and thought I'd put it up in case it was useful. I think the mana was fine, though I had a time or two that I couldn't play a Lord of Atlantis because of my Skycloud Expanse, so I might try it without those. Happy playing!
3 Ponder
3 Hoofprints of the Stag
1 Remand
4 Standstill
4 Sage's Dousing
Creatures
4 Cursecatcher
4 Silvergill Adept
4 Lord of Atlantis
4 Merrow Reejerey
3 Mulldrifter
2 Reveillark
6 Island
4 Mutavault
4 Hallowed Fountain
4 Wanderwine Hub
2 Skycloud Expanse
4 Mystic Gate
2 Aquitect's Will
2 Teferi's Moat
1 Psionic Blast
2 Words of Worship
1 Teferi, Mage of Zhalfir
2 Pact of Negation
2 Mirrorweave
3 Sower of Temptation
The deck is basically an amplification of the card-drawing capability inherent of merfolk, and folds in tempo advantage with Standstill and recursion with Lark.
MU 1: Saprolings...
This deck was able to chump block for about 6 turns each game before merfolk charged to victory. I don't think my life total moved from 20 either game. 2-0.
MU 2: Mono-white Equipment...
This deck actually held up for awhile game 2 after it boarded in and produced multiple copies of Shield of Duty and Reason. A 6/6 Shielded creature with Vigilance beat on me for a couple of turns before I could overwhelm this deck with an army of merfolk and incredible card advantage. 2-0
MU 3: Tron
Game 1 the merfolk spotted an Island on the far side of the table and ended the game in 5 turns. Game 2 I boarded in Pact of Negation. Thinking I was well in control of the game with a small but competent merfolk force, I played Pact of Negation to counter Razormane Masticore. Opponent smoothly proceeds to play, and use, Mindslaver, and decides it would be best for me to lose the game the following upkeep. I humbly oblige. Game 3 we are both at 4 life, and Opponent has 2 Razormane Masticores in play with 1 card in hand. On my turn, I evoke out Lark, bringing back 2 Mulldrifters, but I'm tapped out. Had I just played out Lark for normal cost I would have won this game, but instead my opponent loses 1 Masticore, kills a Mulldrifter, draws into O-Ring and swings for the win. Oh well, it wouldn't be the last time we play today... 1-2
MU 4: UB Faeries
Game 1 I don't hold a Lord of Atlantis but decide to keep anyways. Opponent Mulligans to 6, 5, 4, 3, then concedes. Game 2 I do draw 2 Lord of Atlantis, and the game is over almost before it begins since this particular Faerie deck actually has some Islands. 2-0
MU 5: Tron
Same Tron, but this time the games were much easier. First game I led with a Cursecatcher, Standstill, then dropped a Mutavault and forced opponent to bust Standstill to catch up, which they never did. Game 2 was similar, with an early Hoofprints of the Stag followed by a Standstill, which I find both comical and scary. Standstill was really good on this night, and though I rarely drew into Hoofprints I was really glad to have it, since it plays like a gargadon on Steroids in this deck. This time Pact wasn't met by Mindslaver and I rolled to an easy win. 2-0
The deck worked well because even after dropping an early army I had a full hand of cards and lots of options. I probably Pondered ten times and shuffled in all but one case. I had fun and thought I'd put it up in case it was useful. I think the mana was fine, though I had a time or two that I couldn't play a Lord of Atlantis because of my Skycloud Expanse, so I might try it without those. Happy playing!
-2 Hoofprints of Stag
+2 Vedalken Shackles
-1 Sage's Dousing
+1 Remand
-2 Reveillark
+2 Aquitect's Will (Cantrip is always nice)
I have a similar version of your deck that performed quite well in legacy, might wanna take a look for ideas.
UW Stillfolk: http://forums.mtgsalvation.com/showthread.php?t=122369