Katara, Bending Prodigy

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.

"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 an artifact or creature you control has a mana ability with {T} 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.