Step 1: Prepare the shrimp by making 3-4 shallow incisions along the inner curve of each shrimp's belly to prevent curling. Gently press down on the back to straighten. Pat the shrimp thoroughly dry with paper towels.
Step 2: In a large bowl, combine the tempura batter mix with 1 cup of ice-cold water. Whisk lightly until just combined; it's okay if there are some lumps, as overmixing will develop gluten and lead to a less crispy batter. Keep the batter as cold as possible, even placing the bowl over an ice bath.
Step 3: Pour the neutral oil into a large, heavy-bottomed pot or deep fryer to a depth of 2-3 inches. Heat the oil over medium-high heat until it reaches 350-375°F (175-190°C). Use a kitchen thermometer to monitor the temperature.
Step 4: Once the oil is hot, dip each shrimp into the cold tempura batter, ensuring it's lightly coated. Allow any excess batter to drip off.
Step 5: Carefully lower 3-4 battered shrimp into the hot oil, being careful not to overcrowd the pot. Fry for 2-3 minutes, flipping once, until the tempura is golden brown and crispy.
Step 6: Using a slotted spoon or spider, remove the fried shrimp and transfer them to a wire rack set over a baking sheet lined with paper towels to drain any excess oil. Optionally, sprinkle lightly with sea salt immediately.
Step 7: Allow the oil temperature to return to the ideal range before frying the next batch. Repeat the process with the remaining shrimp.
Step 8: Serve the easy shrimp tempura immediately with lemon wedges, traditional tempura dipping sauce (tentsuyu), or spicy mayo.