茶水 can refer to two related things: (1) tea that has been brewed and is ready to drink — not necessarily weak, just ordinary tea; (2) plain boiled water that's been prepared for making tea (common in workplaces or train stations where hot water dispensers are labeled 茶水). In daily conversation it usually means 'tea' as a beverage, especially when offered to guests or in a casual setting. The English gloss 'weak infusion' is misleading — 茶水 covers all strengths.