A large mixing bowl — big enough to hold 8–10 cups of watermelon without making a mess when you toss it
A good sharp knife — the most important tool here. A sharp knife makes cutting watermelon clean and easy; a dull one makes it a wrestling match.
A chopping board — a large one. Watermelon is generous in size and juice.
Measuring cups — for portioning the watermelon if you want precision
Measuring spoons — for the mint
A citrus juicer or reamer — optional, but squeezing two limes by hand without one is moderately annoying