In a plastic bag, toss the shrimp with the seasoned flour to coat them, transfer the shrimp to a large sieve, and shake off the excess flour. In a kettle heat 1 inch of the oil over moderately high heat until it registers 375 degrees on a deep-fat thermometer and in it fry the shrimp in batches, stirring occasionally, for 1 minute, or until they are just cooked through, transferring them as they are fried to paper towels to drain.
In a bowl stir together the gingerroot, mayonnaise and mustard until the mixture is well combined and spread 1 tablespoon of the ginger mayonnaise on each pieces of toast. Arrange the radishes, alfalfa sprouts and shrimp decoratively on the toast and tuck a watercress sprig into each sandwich. Stir the lemon juice into the remaining mayonnaise and serve the sandwiches with the mayonnaise drizzled over them.