Silvergill Adept has proven itself time and time again to be an amazing card in any merfolk deck. +0 is a whole lot better than -1, and it can give out counters and be buffed while searching for the cards that perform those tasks. Standard, Modern, Legacy, Vintage, EDH, and now Pioneer play.

As for Hinterland Harbor , I may play it as a 1 or 2 of, but with so little lands that are forests or islands, I don't see it coming into play untapped very early in the game. Yavimaya Coast acts very similar to Hashep Oasis when it was legal in standard, except coast taps for both colours and .

stern dismissal is strictly better than unsummon :)

If you're running 31 merfolk, including 3 copies of Realmwalker, you may want to consider moving a couple copies of Collected Company to the sideboard. The number of merfolk tokens you could make from Deeproot Waters would help with the first ability of Kumena, Tyrant of Orazca. Thus, if you don't have a merfolk through Realmwalker on top of your library, you can draw cards until you get a Collected Company or another merfolk. With 3/4 of your creatures 2cmc or less, I think its workable.

Looks great.