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:
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.
Some say you should only follow this custom for 11 months after the loved one's death, similar to how someone says kaddish for a relative for 11 months, lest you imply they were the worst type of person and need the 12 months in gehenna before entering olam haba.