Into large Dutch oven, pour oil to depth of 1 1/2 in. Heat to 375ºF. on deep-fat thermometer.
On waxed paper, mix flour, masa harina, 1/2 tsp each salt and cayenne. On second sheet, mix cornmeal and remaining salt and cayenne. In small bowl, whisk eggs and water to blend.
Gently rinse crabs under cold water. Lightly coat with flour mixture. Dip crabs, 1 at a time, into egg; coat in cornmeal mixture.
With tongs, lower crabs, 2 at a time, into hot oil. Fry 3 to 4 minutes, turning once, until golden and crisp. Drain on paper towels.
Spread each roll half with 1 Tbsp tartar sauce. Scatter 1/2 cup lettuce over bottoms; top each with 3 slices tomato. Top each tomato slice with a pickle slice. Top each with a fried crab and roll top, tartar sauce slide down. Serve with pepper sauce and lemon.
Makes 4 sandwiches