Isn't Martha's but from Lap of Luxury cocktails

2 oz. Plymouth gin (I figured go with what the recipe calls for, but you can use a different gin if you'd like)
1 1/4 oz fresh squeeze Pink grapefruit juice
1/2 oz. Elderflower cordial
1 dash of simple syrup (mix equal parts boiling water and sugar)
1 slice of watermelon

1. To a shaker filled with ice add gin, elderflower cordial, watermelon wedge, simple syrup and pink grapefruit juice.
2. Shake vigorously to break up the watermelon.
3. Strain into a tall glass filled with crushed ice. Garnish with a watermelon ball and a slice of lemon.

Makes one cocktail

I had a cocktail on the ship the other night with cranberry juice, grapefruit juice and vodka. Was very good.