Combine beer, water, and garlic in the pot. Stir to combine.
Add corn, potatoes and sausage. Sprinkle the cajun seasoning (if using) and 1 tbsp of the Old Bay and over the top.
Secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOK function*. Use the +/- buttons and program the Instant Pot for 4 minutes.
When the time is up, quick-release the remaining pressure.
Add shrimp and sprinkle lemon juice and remaining 1 tbsp Old Bay over the top. Replace the lid and make sure the pot is still set to WARM. The residual heat will cook the shrimp. Check for doneness after 5 minutes.
Use a slotted spoon to remove corn, potatoes, sausage and shrimp to a serving platter.
Mix together butter and garlic powder. Pour butter mixture over the platter or serve on the side for dipping.
Garnish with additional Old Bay and/or cajun seasoning. Serve warm.
Notes
*If your Instant Pot does not have a MANUAL function, use the Pressure Cook function.