In a large pot, combine the water, sliced lemons, chili flakes, garlic, and salt.
Bring the poaching liquid to a gentle boil over medium-high heat.
Add the calamari and baby squid, cooking for 1–2 minutes until just tender.
Remove the calamari with a slotted spoon and transfer it to a large mixing bowl.
Add the shrimp and mussels to the hot poaching liquid.
Turn off the heat, cover the pot, and allow the seafood to poach for 4–5 minutes, or until the shrimp are pink and the mussels have opened.
Remove the shrimp and mussels with a slotted spoon and add them to the bowl with the calamari. Discard any mussels that do not open.
In a separate bowl, whisk together the olive oil, lemon juice, salt, oregano, and black pepper until well combined.
Pour the dressing over the seafood.
Add the chopped celery and red onion.
Gently toss everything together until evenly coated.
Cover and refrigerate the Easy Seafood Salad for at least 30 minutes to allow the flavors to develop.
Serve chilled or slightly cool with fresh lemon wedges and chopped parsley if desired.