Slice the watermelon in half lengthwise, then scoop out the flesh from one half while leaving about 1/2 inch of red fruit still attached to the rind. This creates a shell with enough structure to support the gelatin. Set the hollowed watermelon half on a large serving tray or baking sheet, using rolled towels around the base to stabilize it and keep it perfectly level.

Combine both the red flavored gelatin and all the unflavored gelatin packets in a large mixing bowl. Pour the boiling water over the gelatin mixture and stir continuously for about 2 minutes until every granule is fully dissolved. Allow the mixture to cool for approximately 5 minutes until it feels just warm to the touch.

Add the vodka and fresh lime juice to the slightly cooled gelatin mixture, stirring thoroughly until everything is well incorporated. Pour this mixture slowly and carefully into the hollowed watermelon shell, filling it nearly to the rim.

Move the filled watermelon (still on its tray) to the refrigerator and chill for 3 to 4 hours until the gelatin has set completely and feels firm when touched.

Take the watermelon out of the refrigerator and use a sharp knife to slice it into wedges or slices, just as you would cut a regular watermelon. For best results, chill your serving plates or glasses beforehand to help keep the gelatin firm. Serve right away.