Not so long ago, running water on tap was not all that common a thing. This was especially the case for folks living in poor or rural areas -- like your basic old-time hoodoo practitioner was probably like to be.
The habit of finishing a traditional hoodoo bath spell by way of chucking the water out into a road or river probably seemed more logical when a "bath" didn't mean a giant porcelain tub full of liquid. It probably meant a sponge and a bucket.
This post over at the Gibson Girl's Blog has some authentic old time instructions for a spongebath (not hoodoo-specific, but I doubt there was a ton of variation other than that a hoodoo practitioner would scrub up or down as needed for the spell.) Perfumes and scented herbs apparently were even still added when using this washbucket method -- maybe this is why some old spells instruct to use what seem pointlessly miniscule amounts of oils or soaps in the baths ("three drops" etc.) In any event, have fun and use what you can from this data.