Several biblical regulations specify that full immersion in water is required to regain ritual purity after ritually impure incidents have occurred. Most forms of impurity can be nullified through immersion in any natural collection of water. Some require “living water,” such as springs or groundwater wells. Living water has the further advantage of being able to purify even while flowing as opposed to rainwater which must be stationary in order to purify. Discoloration or contamination of the water can invalidate the water for immersion.
In Judaism, a “Mikva” ritual bath is usually constructed especially, as the household or other community water sources do not have the quantity and kind of water required. A mikvah is used in the following circumstances:
1) by Jewish women to achieve ritual purity after menstruation or childbirth (Leviticus 15:5-10, 19-27)
2) by Jewish men to achieve ritual purity (Leviticus 15:5-10, 19-27)
3) by Jewish men or women after discharges (Leviticus 15:13,16) or leprosy (Leviticus 14:6-9)
4) by Jewish men or women after contact with a corpse or grave (Numbers 19:19)
5) by Jewish priests when they are being consecrated (Exodus 40:12)
6) after mistakenly eating meat from an animal that died naturally (Leviticus 17:15)
7) for utensils used for food captured in battle (Leviticus) or of non-Jewish manufacture (Rabbinic)
8) as part of a traditional procedure for conversion to Judaism (Rabbinic)
In Islam, the requirements for ritual bath are the same as in Judaism, including the preferred use of “living water,” such as springs or groundwater wells and the prohibition of discoloration or contamination. There is no minimum quantity of water, and leniency is given to using tap water where its source is springs or groundwater wells, even if it has been stored in retaining tanks before being used. Ghusul is required in the following circumstances:
1) by Islamic women to achieve ritual purity after menstruation or childbirth
2) by Islamic men to achieve ritual purity
3) by Islamic men or women after discharges
4) After contact with a corpse or grave
7) for utensils used for food captured in battle or foreign manufacture *
* There is a haddith (a saying or an act or tacit approval or disapproval ascribed either validly or invalidly to the Islamic prophet Muhammad) that requires this, but I do not know if its is practiced by any Madh’hab (Muslim school of law).
In Christianity, there are no requirements for the ritual bath, although, based on the accounts of John the Baptist, running river water is preferred (specifically the Jordan river). However regular water, standing or springs or groundwater can be used. Baptism is only required in the following circumstance:
1) as part of a traditional procedure for “conversion” (actually initiation) to Christianity, either as a baby (in place of circumcision), or as an adult to express free-choice acceptance of Christianity.
(Disclaimer: I wrote this informally from memory, please verify any statements before relying on what is written here)