Combine chicken breasts, water, salt, and pepper in a multi-functional pressure cooker (such as Instant Pot(R)). Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 20 minutes. Allow 10 to 15 minutes for pressure to build. Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Shred the chicken using 2 forks. Store shredded chicken in an airtight container with the liquid to help keep it moist.