Cut a thin slice off bottom of watermelon, so it will set level. Scoop melon part out of watermelon, within 2 inches from bottom to form bowl. Remove seeds from melon and cut into bite-size pieces.
Cut large scallops around edge of melon bowl. Cover and chill. Peel and seed other melon and cut into bite-size pieces. Combine fruits. Place melon on a serving plate with rim. Spoon fruits into bowl. Chill until serving time. Garnish with mint.