Those who dive into death magic know how strongly death is connected with the earth. Bodies are buried and decomposed within the earth. Ancient Sumerians believed that the soul could not travel to the Netherworld until the body was buried underground. The ancient Greeks recorded passages to the Underworld which included deep caves. Earth is the most prominent element in death magic; but another element that is equally prominent is water.
Just as earth connects to the Underworld in ancient mythology, so does water. You’ve probably heard of the river Styx carrying souls into the Greek Underworld, along with the five other rivers that make up its geology. Entrance into the ancient Egyptian afterlife was granted upon a passage downriver on Ra’s boat. In Mesopotamian mythos, we see a contradiction where some sources depict a soul traveling to the afterlife by road, whereas others travel by river. There are more examples, but to keep the post short I’ll paraphrase: Where there was a land of the afterlife, there was water.
Because of this, one of the most well-documented methods of Roman necromancy is scrying with water. The water was placed into a brass (or sometimes black) bowl, usually with a drop of oil. Green olive oil was most common, although some magicians may have included alcohol or blood instead. The source of the water, too, was very specific. Some sources claim that spring water is most effective to contact the dead, while others claimed river water worked best.
Some records of Underworld travel and hedgecrossing require the magician to either float down a river, or cross a river to enter the afterlife. Cleansing before rituals usually involved spring water for both full body cleaning and fasting. Some forms of spiritual travel require the magician to be submerged as well.
If this topic interests you, I highly recommend researching mythos of the underworld, necromancy and funeral processions from the craft you pull inspiration from.