Pat the fish dry with paper towels. In a large, deep pan over medium-high heat, heat the oil until hot, but not smoking, or when it reaches a temperature between 360°F and 375°F on a deep-fat thermometer. You can test the oil for readiness by dropping in a piece of bread and if it sizzles immediately it is ready.
Dredge the medium size whole fish into the flour and toss to coat. Tap the fish on the side of the bowl to remove any excess flour. Drop the fish into the hot oil and fry until lightly golden on both sides, crispy, and cooked through. Using a wire skimmer or two forks, transfer the fish to a paper towel lined plate to drain while you prepare the remaining fish and seafood.
Dredge the small fish into the flour and toss to coat. Place the fish in a fine-mesh sieve and gently shake away any excess flour. Working in batches, fry the fish until lightly golden on both sides and cooked through, about 1 to 2 minutes. Using a wire skimmer, transfer the fish to a paper towel lined plate to drain while you prepare the remaining fish.
Working in small batches, dredge the squid into the flour and toss to coat. Place the squid in a fine-mesh sieve and gently shake away any excess flour. Fry the squid, one scant layer at a time, until lightly golden and crispy, about 1 1/2 to 2 minutes. Using a wire skimmer, transfer the squid to a paper towel lined plate to drain while you prepare the remaining squid.
Repeat this process for the shrimp, frying the shrimp until lightly golden on both sides, about 2 1/2 to 3 minutes. Arrange the fish on a serving platter and season with salt. Garnish with lemon wedges and serve hot.