Cut watermelon in half and remove watermelon from rind; the easiest way to do this is with a melon baller. Place melon balls into a large mixing bowl. Save the rind to use as the salad bowls for a pretty presentation, scraping out to the same depth all over and cutting a small slice from the bottoms of the outside rinds so that each half will sit evenly on a table.
Crumble feta cheese over the watermelon, chiffonade the mint (roll leaves into a cigar and slice thinly across the 'cigar' to get ribbons of mint) and sprinkle over watermelon. Grind pepper to taste over all.
To make the dressing, blend or shake the olive oil and vinegar in a jar until emulsified. Pour over watermelon and toss gently. Chill for 30 minutes, spoon carefully into reserved watermelon rind 'bowls' and serve.