Secret of Bloodbending

"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."

Controlling a player doesn't allow you to look at that player's sideboard. If an effect instructs that player to choose a card from outside the game, you can't have that player choose any card.

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.

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.

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.

If the targeted player skips their next combat phase or turn, you'll control the next combat phase or turn the affected player actually takes.

In a Two-Headed Giant game, gaining control of a player causes you to gain control of each player on that team.

Multiple player-controlling effects that affect the same player overwrite each other. The last one to be created is the one that works.

The player you're controlling is still the active player during that turn.

While controlling another player, you also continue to make your own choices and decisions.

While controlling another player, you can see all cards in the game that player can see. This includes cards in that player's hand, face-down cards that player controls, and any cards in that player's library the player may look at.

While controlling another player, you make all choices and decisions that player is allowed to make or is told to make. This includes choices about what spells to cast or what abilities to activate, as well as any decisions called for by triggered abilities or for any other reason.

You also can't make any choices or decisions for the player that would be called for by the tournament rules (such as whether to take an intentional draw or whether to call a judge).

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.

You can use only the affected player's resources (cards, mana, and so on) to pay costs for that player; you can't use your own. Similarly, you can use the affected player's resources only to pay that player's costs; you can't spend them on your costs.

You can't make any illegal decisions or illegal choices—you can't do anything that player couldn't do. You can't make choices or decisions for that player that aren't called for by the game rules or by any cards, permanents, spells, abilities, and so on. If an effect causes another player to make decisions that the affected player would normally make (such as Master Warcraft does), that effect takes precedence. In other words, if the affected player wouldn't make a decision, you wouldn't make that decision on that player's behalf.

You can't make the affected player concede. That player may choose to concede at any time, even while you're controlling that player.

You only control the player. You don't control any of that player's permanents, spells, or abilities.