The Didache, or Teaching of the Twelve Apostles (an early Christian text not part of the New Testament) says of baptism (chapter 7):
On the subject of baptism, baptise thus: after having taught all that precedes, baptize in the name of the Father and of the Son and of the Holy Spirit, in living water. If for some reason you do not have living water, baptize in other water; and if you are not able to in cold water, in warm water. If you do not have enough of one or the other, pour out water three times on the head, in the name of the Father and of the Son and of the Holy Spirit. Before the baptism, let the baptizer, the baptized and others who can, observe first a fast; as for the baptized, you must enforce a fast beforehand for one or two days.
"Living water" is usually assumed to mean "running water", like the river Jordan in which Jesus was baptized.
What is the reason for preferring running water over still water, and cold water over warm water?
I know some people who were baptized in the sea; would that count as living water, or does the term only refer to rivers? (It certainly met the preference for cold water, by all accounts.)