If a spell is exiled this way, it's removed from the stack and thus will not resolve. The spell isn't countered; it is exiled. This works against a spell that can't be countered.

If an artifact or creature you control has a mana ability with in the cost, activating that ability while casting a spell or activating an ability with waterbend will result in the artifact or creature being tapped before you pay the spell's costs. You won't be able to tap it again for waterbend. Similarly, if you sacrifice an artifact or creature to activate a mana ability to pay a cost with waterbend, that artifact or creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for waterbend.

Even if the cost of the spell or ability that waterbend is part of increases or decreases, you still may only tap artifacts or creatures to pay for costs up to the amount of the generic mana in the waterbend cost.

To airbend one or more permanents and/or spells means to exile each of those objects. For each card exiled this way, for as long as it's exiled, its owner may cast it from exile by paying rather than paying its mana cost.

If the spell you cast has in its mana cost, you must choose 0 as the value of X.

"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."

Lands exiled this way cannot be played from exile. (Lands that were animated by earthbend will be returned to the battlefield tapped when they're exiled.)

Since you are using an alternative cost to cast a spell from exile this way, you can't pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

If a waterbend cost is part of the total cost to cast a spell or activate an ability, you may tap an untapped artifact or creature you control rather than pay one generic mana in that total cost any number of times up to a maximum of the amount of generic mana in the waterbend component of that total cost.

Tokens exiled this way will cease to exist and cannot be cast.

You can tap any untapped creature or artifact you control to pay a waterbend cost, even one you haven't controlled continuously since the beginning of your most recent turn.