MTG Keyword Rules — Comprehensive Rules Glossary

“Waterbend [cost]” means “Pay [cost]. For each generic mana in that cost, you may tap an untapped artifact or creature you control rather than pay that mana.”

If a waterbend cost is part of the total cost to cast a spell or activate an ability (usually because the waterbend cost itself is an additional cost), the alternate method to pay for mana described in rule 701.67a may be used only to pay for the amount of generic mana in the waterbend cost, even if the total cost to cast that spell or activate that ability includes other generic mana components. Example: Spirit Water Revival is a spell that costs {1}{U}{U} and says, in part, “As an additional cost to cast this spell, waterbend {6}.” As you cast it, you may tap up to six artifacts and/or creatures you control to pay the for the {6} in its waterbend cost, but you can’t tap any artifacts or creatures to pay for any part of the {1}{U}{U} mana cost.

An ability that triggers whenever a player waterbends triggers whenever that player pays a waterbend cost, regardless of how they paid that cost.