I have added this to my page, “Writing: A Guide“, as section IV.B.4.
Most newspapers and magazines follow the convention of forming the possessive of a word ending in “s” by putting an apostrophe after the “s”; for example:
Dallas’ (for Dallas’s)
Texas’ (for Texas’s)
Jesus’ (for Jesus’s)
This may work on a page or screen, but it can cause ambiguity if carried over into speech*. (Warning: I am about to take liberties with the name of Jesus and the New Testament, about which I will write as if it were a contemporary document. Read no further if you are easily offended.)
What sounds like “Jesus walks on water” could mean just what it sounds like: a statement about a feat of which Jesus is capable or is performing. But if Jesus walks on the water more than once, it could refer to his plural perambulations: “Jesus’ walks on water”**, as it would appear in a newspaper.
The simplest and best way to avoid the ambiguity is to insist on “Jesus’s walks on water”** for the possessive case, and to inculcate the practice of saying it as it reads. How else can the ambiguity be avoided, in the likely event that the foregoing advice will be ignored?
If what is meant is “Jesus walks on water”, one could say “Jesus can [is able to] walk on water” or “Jesus is walking on water”, according to the situation.
If what is meant is that Jesus walks on water more than once, “Jesus’s walks on water” is unambiguous (assuming, of course, that one’s listeners have an inkling about the standard formation of a singular possessive). There’s no need to work around it, as there is in the non-possessive case. But if you insist on avoiding the ‘s formation, you can write or say “the water-walks of Jesus”.
I now take it to the next level.
What if there’s more than one Jesus who walks on water? Well, if they all can walk on water and the idea is to say so, it’s “The Jesuses walk on water”. And if they all walk on water and the idea is to refer to those outings as the outings of them all, it’s “The water-walks of the Jesuses”.
Why? Because the standard formation of the plural possessive of Jesus is Jesuses’. Jesusues’s would be too hard to say or comprehend. But Jesuses’ sounds the same as Jesuses, and must therefore be avoided in speech, and in writing intended to be read aloud. Thus “the water walks of the Jesuses” instead of “the Jesuses’ walks on water”, which is ambiguous to a listener.
* A good writer will think about the effect of his writing if it is read aloud.
** “Jesus’ walks on water” and “Jesus’s walks on water” misuse the possessive case, though it’s a standard kind misuse that is too deeply entrenched to be eradicated. Strictly speaking, Jesus doesn’t own walks on water, he does them. The alternative construction, “the water-walks of Jesus”, is better; “the water-walks by Jesus” is best.