Además, traigo agua para todos.

Además means “in addition,” “besides,” or “what’s more.” It introduces an extra point on top of what was just said. También means “also/too” and is a bit lighter and more neutral. In many contexts both work, but además often sounds a little more emphatic or argumentative than también. Example:

• También traigo agua = I also bring water.
• Además, traigo agua = What’s more, I’m bringing water.

Spanish contrasts traer (to bring toward the speaker/listener’s location) and llevar (to take away to another place). Use:

• Traigo when the water is being brought to where the conversation is centered (e.g., you’re arriving to the group).
• Llevo when you are taking it somewhere else. Note: In much of Latin America, traer can also mean “to have/carry with you right now” (e.g., ¿Traes agua? = “Do you have water (on you)?”), which fits this sentence well.

Agua is grammatically feminine. It uses el (not la) in the singular article only to avoid the clash of two adjacent stressed “a” sounds: el agua. Agreement stays feminine:

• el agua fría (feminine adjective)
• Plural goes back to las: las aguas frías. Pronouns agree with the real gender: ¿El agua? Sí, ya la traigo.

In context it can mean either. To be explicit:

• “everyone” (general): para todos, para todo el mundo.
• “all of you” (you plural): para todos ustedes (Latin America).

If “water” is already known, use a direct object pronoun for it and an indirect object for the people:

• Se la traigo a todos.
  Here, la = “the water” (feminine), and se replaces les before la (you can’t say les la). You can also say: Les traigo agua a todos when you keep “agua” explicit.