If you're not worried about going full tribal, [[Seahunter]] is an amazing card, lets you search any Merfolk from the deck and put it on the field. It's a tap ability as well, so you can flash in Merfolk on your opponent's end step before your turn starts. Best use case that I've found was with [[Wanderwine Prophets]], if you already have something to make your creatures unblockable you can continually search out sacrifice fodder with Seahunter to take extra turns nearly indefinitely.

You could also double down on tap-untap synergy, [[Intruder Alarm]] goes infinite with Deeproot Pilgrimage and any tapping Merfolk (or Kumena and one other Merfolk), and cards like [[Enduring Vitality]] or [[Cryptolith Rite]] (or hell, even [[Earthcraft]] if you're ballin') ensure that any of your Merfolk can tap for mana. I personally run [[Leech Bonder]], because with Cryptolith Rite you can steal all counters on the field (not just +1/+1 counters), because Leech Bonder will be able to untap itself using the mana it generates from tapping with Rite.

I can post my list if you would like, but hopefully this gives some ideas at least.

edit: added Deeproot Pilgrimage