Pour the water into the bottom of the Instant Pot and arrange the handled trivet over that. (This is the trivet that comes with your pressure cooker.)
Place the boneless chicken breasts on top of the trivet and sprinkle them generously with salt and pepper. Secure the lid and move the steam release valve to Sealing. Select Manual or Pressure Cook (depending on your machine) and cook at high pressure for 12 minutes. If your chicken breasts are on the smaller side, you can cook them for just 10 minutes, and if they seem especially large, you can cook them for 15 minutes.
When the cooking cycle ends, quickly release the pressure by moving the steam release valve to Venting. When the floating valve in the lid drops, it's safe to remove the lid.
Use tongs to transfer the chicken to a cutting board, and let it rest for at least 5 minutes before slicing to help it retain its juices. Use the chicken right away, or store it in an airtight container in the fridge for 3 to 4 days.
To Cook Frozen Instant Pot Chicken: Pour 1/2 cup of water into the bottom of the pot and place the frozen breasts on top of that. (No trivet necessary.) Make sure the frozen breasts are not stuck together, as they won't cook evenly otherwise. Cook at high pressure for 12 minutes, then let the pressure naturally release for at least 10 minutes before venting and removing the lid.