Preheat oven to 425°F (220°C). In a bowl, toss potato slices, 1 tsp (5 mL) oil, the cumin and 1/2 tsp (2 mL) salt to coat. Spread slices in a single layer on a large parchment paper-lined baking sheet. Set aside.
Season fish with remaining salt and the cayenne pepper. In a shallow dish, beat eggs and water with a fork. Spread rice cracker crumbs on a large plate. Dip each fillet into egg mixture, then coat in cracker crumbs. Arrange fillets on a baking sheet and drizzle with remaining oil.
Bake sweet potatoes 15 to 20 min., turning once halfway through. At the same time, bake fish 8 to 12 min. or just until opaque. (Fish can rest a few min. while sweet potatoes finish baking.) Serve fish and sweet potato chips with lemon wedges and malt vinegar, if desired.