Selling cold bottled water on a hot day is an easy way to make some quick cash, but be forewarned, depending on the laws where you live, it might not be a legal one.
True, most U.S. cities have laws on the books that make it less than totally legal to sell bottled water without a vendor's permit and health department license. By that logic, lemonade stands are also illegal (and they are). But unless your local police routinely imprison Girl Scouts, your makeshift water operation will likely go unnoticed.
They key to making money in any retail venture is to buy low and sell high. A case of 24 16.9-oz (500-milliliter) bottles of water sells for around $7 in big box stores like Wal-Mart and Target. If you have a Costco or Sam's Club card, you can get them for even less in bulk. If you can sell each bottle for $1, you're looking at a quick $17 profit in maybe an hour.
Location is key. If the weather's hot and your product is cold, you can find customers just about anywhere that people gather: a neighborhood park, the entrance to a subway station, bus stops, or outside sporting events. Selling on the side of a busy road can be lucrative, but potentially dangerous and more likely to draw the attention of the law.