Guide des Méandres

If a card in a graveyard has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

You don't choose a target for Meanders Guide's ability at the time it triggers. Rather, a second "reflexive" ability triggers when you tap another untapped Merfolk you control this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.