Buy one bottle you will actually carry, stainless steel, aluminium or a tough reusable plastic, and fill it from the tap or a fountain. Stop buying single-serve water unless tap water is unsafe where you are.
A 2025 study in Environmental Research found making one aluminium bottle emits about 13.1 kg CO2e against 0.28 kg for a single HDPE bottle, but over a year of drinking a litre a day the reusable bottle vastly outperforms the 730 PET bottles it replaces, and the yearly impact of a refillable bottle is negligible regardless of material. A 500 ml PET bottle weighs about 12 g, so four bottles a week avoided is roughly 2.5 kg of plastic a year.
The catch: the bottle only wins if you use it. A drawer of abandoned reusables is worse than the plastic they were meant to replace.