Thought I would share my BO1 version that is a little unorthodox but so far has been great.
Creatures
4x
Mist-Cloaked Herald - thought about cutting for pterry but my build has less instants than most monoU builds so sticking with mer-folk singer for now
4x
Siren Stormtamer - My all star early wizard
4x
Merfolk Trickster - beast, target those
wildgrowth walkers before op goes exploring
4x
Surge Mare - RDW's and Golgari's nightmare. He is mana intensive to work with at times but i have found he is the thopter copter this deck needs that does more than opt or chart a course could ever do vs aggro.
4x
Tempest Djinn - The beat stick, never play on curve unless you have no other choice
2x
Exclusion Mage- Great card to out tempo aggro and a body vs control. Only 2 here but has been good when I need him.
Spells
4x
Curious Obsession - key card
4x
Dive Down - I play the deck vs RDW often and this card allows you to win.
4x
Spell Pierce - same as above but great vs any on curve control sweeper or teferi. Good vs enchants and that early nexus as well.
4x
Wizard's retort - Mostly played for UU and I don't run creature counters so pretty much my only answer to serious threats.
1x
Syncopate - Only a 1 of but has been great every time i draw it. Some cards just need to be exiled (nexus, chemister's insight)
Lands
21x Islands - have thought about going up to 22 because I play without a lot of the filtering other MonoU plays with but thats what surgemare is good for but he also needs mana to attack.
Fun deck to play and I am hoping to play it more often as I just upgraded to a new machine. Now if only teferi could help me out with slowing down time so i could actually play more.