I didn't find this all that sweet. Could be because I made a 1:1 watermelon syrup. I made this with 50 ml fresh juice. Had potential... Tried it again this way: 60 ml tequila, 40 ml juice, 20 ml lime, 20 ml syrup, 2 drops saline. It's getting there.
Nice and refreshing. Found it a bit sweet (as I do most Margaritas). The watermelon we had was very sweet so I would suggest a slight taste before adding the sugar syrup to check the sweetness.
Did add 2 drops of Difford's Margarita Bitters which subdued the sweetness some. Would also suggest a splash of cold water was we were not using wet ice.
On the whole, it is perfect for a warm summer afternoon.