Someone told me that one is not allowed to drink plain water during the third meal of Shabbos (Seudah Shelishit). (or at least that there is a minhag not to).
Is this true? And if so, what is the source of the minhag?
There is such a minhag, but the language of the Rema in 291:2 is:
Some say that it is forbidden to drink water between mincha and maariv on shabbos because that is when the souls return to purgatory (reasoning needed?). Therefore, one should not eat seudah shlishis between mincha and maariv, rather he should eat it before mincha.
Yet some say it is better to daven mincha first, and such is the custom in all of these provinces. But one should not drink water from the rivers (reasoning needed?), but in the house it is permitted, and certainly other drinks are permitted.
Some say that it is only forbidden within the 12 months [following the death] of one's father or mother.
And some say that the restriction of drinking water is only on erev shabbos (reasoning needed?)
We see two things from the Rema:
The Aruch haShulchan reads the Rema as 2 opinions: Eat before mincha and eat after mincha. The rest of the Rema is supporting opinions for eating/drinking after mincha- it's only from rivers, it's only water, some say it's only for an avel, some say it's only on Friday eve.