Quick Notes.
Overall not bad, but the deck feels clunky. Metallic Mimic just seemed like it slowed the deck down. Several times in games I was top decking, so more card draw needs to be in the deck or more digging in the library.
Cards that stood out;
Blossoming Defense was an MVP several times.
Unsummon was a great tempo card
Herald of Secret Streams was a wincon in two matches.
Deeproot Waters scared the hell out of opponents who knew exactly what it was and was a prime target of theirs. It went well with Jungleborn Pioneer. During the second match against a Dino Aggro deck, I had 2 Deeproot Waters, 1 Metallic Mimic, and 1 Deeproot Elite in play and kept a great board presence. Dropping Herald of Secret Streams allowed the Merfolk attack wave to go underneath his critters.
I prefer the hard counter of Negate over Spell Pierce for sweepers. Perhaps a couple copies of Heroic Intervention in the sideboard also.
Cards that didn't perform well or were just blah;
Kumena, Tyrant of Orazca while having a great toughness even with a counter, under performed IMO.
Metallic Mimic just felt like it made the deck clunky. Not to say it didn't do what it was supposed to, but the deck didn't feel smooth.
My mana base of; 4x Botanical Sanctum, 6x Forest, 1x Hashep Oasis, 7x Island, 4x Unclaimed Territory didn't seem right through four matches. I kept getting mana screwed on one color or the other.
So, I will update to this; 4x Botanical Sanctum, 2x Evolving Wilds, 5x Forest, 1x Hashep Oasis, 4x Island, 4x Unclaimed Territory, 2x Woodland Stream
Sideboard
-1 River's Rebuke and -1 Spell Pierce for +2 Heroic Intervention
Mainboard
The drive of this deck is Herald of Secret Streams and creatures with counters to go under an opponent's battlefield. Secondary is going wide. So I may be committing a Merfolk sin by -2 Kumena, Tyrant of Orazca, -2 Metallic Mimic for +4 Merfolk Branchwalker and -1 Deeproot Waters for +1 Herald of Secret Streams