Put the jelly into a large jug, pour the boiling water over the top, stirring until the jelly is smooth.
Stir in the vodka if using, or remaining cold water and whisk well to combine. Leave for 1 hour until room temperature, then place in the fridge for 2 hours until just lightly set.
Spoon the wobbly jelly into the shot glasses, putting more in some and less in others.
Make a little hole in the side of the ball of melon with a knife, then spoon some of the raspberry sauce into it.
Carefully drop the melon ball, sauce side up, into each shot glass, allowing a little raspberry sauce to drizzle out as it goes into the jelly - it should sit on top of the semi set jelly.
Fill to the top with the rest of the jelly then return to the fridge until set - at least 2 hours.
Serve at room temperature.