Juice enough limes to equal 1/2 cup of lime juice. (It depends on how juicy your limes are, but typically 4-6 limes is enough. Pour the lime juice into a pitcher and add the squeezed limes.
In a small saucepan, combine the water and sugar. Heat over medium high heat, stirring occasionally until the sugar dissolves. Set aside and cool to room temperature, about an hour.
Remove the squeezed limes from the lime juice and discard.
Add water or sparkling water to the pitcher and stir well. Taste the limeade and adjust to your tastes -- adding more lime juice, guava simple syrup or water as needed.
To serve, fill a glass with ice and pour the guava limeade over the ice. Garnish with lime slices, chunks of guava or a sprig of mint. Enjoy!