This is about Cantonese culture. “Buy water” or “Buy heavenly water” for the father who just passed away at the funeral by the eldest son is ritual in Guangdong and the Cantonese phrase “擔幡買水” or just “買水(buy water)” refers to this ritual. That is why you should not tell or ask a Cantonese to buy water in case this Cantonese is pretty superstitious (Some older Cantonese may be pretty upset while some young ones might never hear about this ritual or phrase so they do not care the phrase “買水”). So to be careful, you can change the wording to “buy some drinks”, or “get some water”. For example, you may say “整幾支礦泉水來” (get some bottles of mineral water) instead.

The following is from Wikipedia “擔幡買水是广东人治喪時的一個儀式，由死者的至親於喪禮中負責。「幡」即長幡燈籠，由死者的長子嫡孫提著，代表引領亡靈升天。「買水」即「買天水」，在大殮前由孝子放一個硬幣於一個盛水的盤中，再以白布沾水，於遺體旁上下擦三次以潔淨亡靈。”

Another similar custom is

You should never buy a clock as a gift for A Cantonese

Actually, you should never do that for any Chinese speakers. Of course, they may forgive you thinking that you may not understand their culture, or they may not.