Katara, Water Tribe's Hope

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.

Katara, Water Tribe's Hope's ability overwrites any effects that previously set a creature's base power and/or toughness. Any existing effects or counters that raise or lower a creature's power and/or toughness continue to apply.

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.