Place salmon in a nonreactive casserole dish. Spread Florida orange and grapefruit zests and thyme evenly over salmon. Sprinkle with Florida orange and grapefruit juices and vodka. Combine sugar and salt. Cover flesh side of salmon with salt and sugar mixture. Cover and refrigerate for 4-5 hours, depending on thickness of fillet (longer for thicker fillets, less time for thinner fillets).
While salmon is curing, prepare Florida Orange Beet Vinaigrette: Grate 2 beets on fine side of cheese grater. Use a piece of cheesecloth to squeeze juice from beets. Reserve juice and discard beet pulp. Combine 1 cup Florida orange juice with beet juice and ¼ cup sherry vinegar. Bring to simmer in medium saucepot and reduce volume by ⅔. Chill and reserve.
Remove salmon from refrigerator and rinse off salt mixture under cold water. Pat dry with paper towels. Remove skin and any dark flesh. Slice thinly.
To serve, lay 4-5 slices of salmon on each plate. Drizzle with Florida Orange and Beet Vinaigrette.
Garnish with watercress leaves and serve with crackers (if desired).