The man left the motor running and after he got out of the car it began rolling down an embankment into the Vilterser-Wangser channel, cantonal police said.
The amateur photographer had forgotten to engage the parking brake in the incident, which occurred on Saturday.
Police issued a photograph of the Skoda car stuck with its wheels fully immersed in the channel, which serves to drain parts of the Rhine Valley in this eastern part of Switzerland.
The fire department responded to the scene and a crane was hired to remove the car.
A police spokesman told Blick newspaper that the cost of the retrieval operation, not likely to be cheap, would be borne by the motorist.
Photo: Saint Gallen cantonal police