Watermelons are huge, and not the easiest fruit to cut. Use this bowl method to cut and store evenly-sized cubes in minutes.
It might need a little practice to get it down, but it's one of the fastest methods we've seen. Slice the watermelon in half, then make about five more cuts around the skinned side of one half, but be careful to not connect the slices and make it fall apart. Then slice deep squares on the flesh side, and dump the cubes out into a bag, bowl, or tupperware.
By contrast, this method (which we also love) involves cutting away the rind from the outside and then the middle into cubes. Pick the one that works best for you and enjoy.
The Easiest, Fastest Way To Cut A Watermelon [This Blew My Mind (YouTube)]