That seems not to be an ideal description of them. They regard him as the last and greatest of the prophets, but I don't think John would recognize them.
They're the last of the ancient gnostics. Baptism is an important weekly ritual. They have a curious sacred book--the Ginza Rba, which is divided into two parts: a Right and Left Ginza, printed on alternate pages upside-down wrt each other. The Right's current version seems to date from "early Islamic" era; the Left's is much older. If you want a sample: The Book of John the Baptist is a collection of "proverbs", such as straightforward ones like "The first of your care is: know your account and then speak." and "The unjust is like a pomegranate, showing a resplendent face from the outside, but inside is full of mold." and some that are confusing "The words of the wise man at the gates are like pearls on a pig." I gather that "at the gates" has to do with the gates of ignorance and evil--did I mention that they are gnostics?
Their funeral ceremonies--The Mandaean Ṭabahata Masiqta--are complex, involving raisins in water, a pigeon, and many rituals, some of which seem vaguely Egyptian (they have an opening of the mouth, etc). This is a diagram of part of the ceremony. The new body is in the Light-world, of course.
Apparently a Mandaean who marries outside the faith is deemed to have left it, and nobody is allowed to convert into it, so it's a strictly ethnic religion at this point--and a persecuted one.
If you are curious about what the ancient pagan rituals were like, you could do worse than read that funeral exposition. It seems so wildly different from what you usually think of when you hear "gnostic" that I wonder if it came from merging two different religions--maybe a long forgotten compromise. "We aren't going to choose between the old cult and the new elite gnostics--we'll do both"