designer water

John GrishamのThe Brethrenを読んでいるとこんな表現が出てきました。
Dinner the night before had been nearby, at a crowded new seafood place two blocks from the Sea Turtle Inn. Given the size of the portions, the prices were outrageous, and that was exactly why Trevor, the newest millionaire in Jacksonville, had insisted they eat there. Of course the evening was on him and he spared no expense. He was drunk after the first martini, and didn't remember what he ate. Wes and Chap had explained that their client did not allow them to drink. They sipped designer water and kept his wineglass full.
文脈からして "designer water" はいわゆるブランドのミネラルウォータを指していると容易に分かります。
・Urban Dictionary: A commercial brand of mineral water, often served or sold under pretentious circumstances or marketing and/or at exorbitant cost.: I went to the cocktail bar with the guys, but as I was drivin' I had to sip designer water all evening.
"designer water" の検索では上記の辞書しか見つかりませんでしたが、"designer" で調べると多くの辞書に次のような説明があります。
・Oxford English Dictionary: [only before noun] made by a famous designer; expensive and having a famous brand name: designer jeans: designer labels: designer water
・Merriam-Webster's Online Dictionary: intended to reflect the latest in sophisticated taste or fashion: designer ice cream: a designer haircut
ミネラルウォータは人間がデザインした訳ではないのにおかしいですね。