When do the up and down controls appear?

They appear after the potato leaves the surface and begins swimming underwater.

How do I enter the low door?

Dive while moving right to the door platform, then use up to rise into the arched entrance.

Mr Potato level guide

medium

Swim right across the surface, dive with the vertical controls, and rise into the low door on the right platform.

Layout: The room is filled with water. The potato starts near the left surface, and the arched door is mounted on a low rectangular platform in the lower-right area.
Goal: Reach the lower-right platform and enter its arched door.
Opening: Hold right to cross the surface until the up/down swim controls appear, then press down to begin the door approach.
Danger Zone: The surface route passes above the low exit. Staying at one depth after reaching the door area leaves the potato misaligned with the platform.
Mechanics: Underwater movement adds up and down controls to the normal left and right controls; the solve requires a deliberate dive followed by a rise into the door.

Treat the level as two phases: surface crossing first, vertical alignment second.
Use the white bubbles as visual depth cues, but prioritize the door platform and the up/down controls.
If the board feels stuck, look for the color with the cleanest path and use that to regain space.

Swim right from the left surface across the sea.
When the vertical controls appear, press down to dive below the surface.
Continue right and adjust depth until the potato is beside the low platform with the door.
Press up to rise into the arched doorway, using left or right to correct the final alignment.
Enter the door to trigger the victory screen.

Staying at the surface all the way across leaves the potato above the low exit.
Using only horizontal movement after the vertical controls appear misses the door's depth.

When do the up and down controls appear?
They appear after the potato leaves the surface and begins swimming underwater.
How do I enter the low door?
Dive while moving right to the door platform, then use up to rise into the arched entrance.