The rain that falls in the area surrounding the canal and Gatun Lake provides the water for the almost 2 million gallons released each and every thime the locks open. This part of Panama is subject to heavy rain at almost any time for short periods.
During our crossing we had bright sunshine resulting in temperatures over 110 degrees at midday. If it rains, you will only be participating in an integral part of the canal operation.. Don't worry about it or let it spoil your crossing if it happens. It's only water.