This one is always checked /cleaned from underneath, because.......... The servo is in the way.
That said, as long as the one under the ECU is clear then you are reasonably safe the plenum cannot retain water as the chamber is a single area.
NB: The one exception to this are the pre production cars that MAY have a mini bulkhead separating the bottom parts of the left and right side of the plenum. (The one S reg I checked some time ago did but my launch model doesn't)