Beat together the cake mix with coconut water. Fill 2 (6-count) mini angel food can pans halfway with batter. Bake until puffed, golden brown, and toothpick inserted in the center comes out clean, 12 to 15 minutes. Invert the pans onto wire cooling racks and cool completely before removing from the pans.
For the frosting: Bring 2 cups water to a simmer in a medium saucepot.
In a large glass bowl that will fit over the saucepot, combine 1/4 cup water, sugar, corn syrup, cream of tartar, and egg whites. Place on top of the saucepot and beat for 7 minutes using an electric hand mixer. The frosting will be smooth, shiny, and form peaks when the mixer is lifted from the bowl. Remove from the heat and slowly beat in the marshmallow cream.
Frost each cake with the icing and sprinkle the tops with coconut flakes.