Captan isn't going to hang around anyway, rain or shine:
What happens to captan outdoors?
The half-life of captan in soil ranges from less than 1 to 10 days (1, 9). See Half-life box.
Captan’s half-life on plants ranges from 3 to 13 days (1).
The half-life of captan is less than one day in water (1).
Captan is stationary to slightly mobile in various soils (1).
This pesticide is toxic to fish and wildlife. For terrestrial uses, do not apply directly to water, to areas where surface water is present or to intertidal areas below the mean high water mark. Do not contaminate water by cleaning of equipment or disposal of wastes. Drift and runoff from treated areas may be hazardous to aquatic organisms in adjacent aquatic sites. This product is toxic to bees exposed to direct treatment. Do not apply this product while bees are actively visiting the treatment area.
Seems to me that water does not reduce its effectiveness, though it may not stay where you put it.