Place the water in a large saucepan over medium heat. Bring to a boil and add the hibisucs flowers. Let steep for 5 minutes.
Strain the mixture into a heat-safe pitcher and stir in the agave. Let sit on the countertop or refrigerate until cool.
Squeeze one of the limes into the pitcher and stir. Drop in the lime slices just before serving and serve over ice.
Notes
You can find dried hibiscus flowers at some major grocery stores (check the bulk bins, and also the ethnic foods aisle). You can also get them on Amazon.