For dressing, combine lime juice, water, oil, sugar, celery seed, and mint in a screw-top jar. Cover and shake to mix well. Set aside.
Cut avocados lengthwise through the fruit around the seed. Separate halves and remove seed. Peel and slice avocado. Line a large platter (about 16 inches in diameter) with romaine and/or salad greens. Place jicama strips in center of platter. Arrange grapefruit, pear, avocado, and grapes, if desired, around jicama. Shake dressing well; drizzle over salad. Makes 12 servings.