I just received a 39 scope cap. It is NOT the correct size. The outside dimension of my 45 VF, with the rubber eye piece, is 53mm. It looks like size 33 (30330) is more appropriate. Shame on me for not measuring first.
Underwater mode (1-fish/3-fish) will 'jump' between 12 mm and 50 mm, but it will also change the aperture and speed to some predetermined settings (not configurable). Using the mode switch to switch from, and back to M will return the settings to the ones you have previously set and leave the lens in the zoom in or out position. Not very user friendly. Better, and easier, way is to set up 'electric zoom'. Hit the left arrow to activate. The use up and down arrows to zoom in or out. Upper right corner of the display will show zoom setting. Aperture and speed will not be affected.