In a heavy 2 to 3 quart saucepan heat the vegetable oil; add the barley and cook the grain, stirring constantly, until it smells toasty. Remove the saucepan from the heat. Averting your face, add the water (it will bubble up furiously) Return the saucepan to the heat, cover and cook for about 2O to 25 minutes or until barley has absorbed the liquid and is just tender. Transfer the barley to a sieve and drain under cold water; pat dry and set aside.
During the time it takes to cook the barley, peel, halve and seed the cucumbers and cut them into 1/2 inch half moons. Halve and seed the zucchini and yellow squash, cut them into 1/2 inch half moons and parboil them for a minute, drain and pat dry.
In the bottom of a mixing bowl combine the mayonnaise, vinegar and dill; season well with salt and pepper. To the dressing add the cooked barley, cucumbers and squash and toss well; adjust the seasoning.
On the outside edge of a serving platter, make a ring of drained sliced pickled beets. Pile the barley salad in the middle and center some watercress leaves in the middle of the salad.