That's the Costco water bottle deposit. The 5 cents on a water bottle is an automatic return, with the same amount as the gas station bottle deposit. It's not cash, and it's not refundable.
The water bottle deposit is for bottles only; it doesn't apply to reusable bottles, such as those made from stainless steel or plastic.
The Costco water bottle deposit is to help reduce litter and waste. We don't want people tossing their empty bottles into the trash, where they can clog up the recycling system and eventually be sent to a landfill.
This deposit also helps make sure that only clean, recyclable bottles are being used.