Preparation

• Cut off both ends of one of the grapefruits. Stand it on one end and cut off the peel and the white pith to expose the flesh. Slice the membrane, letting them fall into a small bowl. Squeeze the juice from the membrane into a 1-cup liquid measuring cup; discard the membrane. Cut the grapefruit segments crosswise into 4 pieces. Drain the pieces and add the juice to the measuring cup. Juice the remaining grapefruit and add enough of it to the reserved grapefruit juice to yield 2/3 cup. Refrigerate the grapefruit segments and watermelon pieces separately.

  In a medium nonreactive bowl, combine the scallops, grapefruit juice, lemon juice, red onion, and habanero. Cover and chill until the scallops are opaque, about 30 minutes.

  Drain the scallop mixture in a fine sieve and transfer to a large bowl. Gently stir in the watermelon, grapefruit, and parsley. Season to taste with salt and pepper. Serve cold on small plates or in bowls.