Combine ingredients, except tonic, in a mixing tin and shake.
Strain into a highball glass over 1 prefrozen whiskey stone.
Garnish with 1 piece of Atlantic kelp across the top and another inside the glass.
Notes
* For Kelp simple syrup: Bring 1 cup water, 1 cup sugar to a boil. Add 2 oz. dehydrated Atlantic kelp and allow to steep. Puree and strain with a chinois. Add another 2 oz. of kelp to strain syrup. Let coo and transfer to a squeeze bottle.