I have not tried this as I don't have much of a snail problem (yet), but I found this receipe in "Great Green book of Garden Secrets" by Jerry Baker:
Super Slug Spray1 1/2 cups of ammonia
1 tbsp. of Murphy's Oil Soap (have no idea what that is)
1 1/2 cups of water
Mix all of the ingredients in a handheld mist sprayer bottle, and overspray any areas where you see slug activity. It doesn't say when is the best time to spray, but I would guess that with the oil part of the spray it is best to do it early am or evening or else you might scorch the foliage. (Oh, and don't use it on lotus leaves, I think they are too sensitive to oil sprays and will scorch)
Let me know how this works if you decide to use it.