Prepare the potatoes: Peel the potatoes (optional; if desired) and roughly cut them into approximately 1-inch cubes.
Add to the Instant Pot: Place the potato cubes in the stainless steel inner pot of the Instant Pot. Pour water over them until they are completely covered (approximately 4 cups).
Cook: Place the lid on the Instant Pot and lock it by turning it clockwise until it chimes. Set the Pressure Release knob to Sealing. Select the Pressure Cook option on High for 4 minutes. Press Start.
Release the pressure: When the Instant Pot beeps to signify the end of the timer, carefully switch the Pressure Release knob from Sealing to Venting to release the pressure (I recommend using a wooden spoon or something similar to protect your hand from any escaping water or steam). Once the stainless steel float valve has dropped, turn the lid counterclockwise to open the pot.
Drain: Using a towel or potholders, carefully lift the inner pot and dump the potato-and-water mixture into a colander over the sink. Let the potatoes drain in the colander for a few minutes.
Mash: Return the drained potatoes to the inner pot. Use a potato masher to mash the potatoes until they are smooth. Add the unsweetened soy milk and garlic powder. Stir well.
For traditional mashed potatoes: Taste and, if desired, add ¼ teaspoon of fine sea salt before serving.
For cheesy rosemary mashed potatoes: Add an additional ¼ cup soy milk, nutritional yeast, rosemary, and an additional ¼ teaspoon garlic powder. Taste and add ¼ to ½ teaspoon fine sea salt, if desired. Serve warm.