Cymopoleia, The Rolling Tide

Whenever a nontoken Merfolk creature enters
the battlefield under your control, it explores.
Whenever an Island enters the battlefield under
your control, you may tap or untap target
creature. If a Merfolk you control is tapped this
way, it explores.
Remove a +1/+1 counter from a Merfolk you
control: Create a 1/1 blue Merfolk creature token
at the beginning of the next end step