As an additional cost to
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) this spell, you may
waterbend“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.
An ability that triggers whenever a player waterbends triggers whenever that player pays a waterbend cost, regardless of how they paid that cost. (CR 701.67) .
You control target opponent during their next combat phase. If this spell's additional cost was paid, you control that player during their next turn instead. (You see all cards that player could see and make all decisions for them.)
ExileTo exile an object, move it to the exile zone from wherever it is. See rule 406, “Exile.” (CR 701.13) Secret of Bloodbending.