how can rain play a soilsport in the camping trip?

Rain makes everything soggy and wet, including the men. During the trip the rain water has soaked all of the food stores, and thus, becomes the main article of diet at supper.

Rainwater is the chief article of diet at supper. The bread is two-thirds rainwater, the beefsteak-pie is exceedingly rich in it, and the jam, and the butter, and the salt, and the coffee have all combined with it to make soup.