Pour the hot water over the hibiscus flowers, then let it steep for 10 to 15 minutes.
Strain, then add the cold water, sweetener, and lime juice.
Chill, then serve over ice.
Watermelon version:
Blend the watermelon until smooth, then strain if you'd like.
Add the water, lime juice, and sweetener, then stir to combine.
Serving: Pour into glasses over ice, then garnish with a lime slice or mint leaf.
You can also make multiple flavors (for example, cucumber lime, pineapple, or strawberry), and as a result, you can create a real "drink bar" at the wedding.