Where the tap is fine. Japan, Korea, Singapore, every European city on the site and New York. Singapore’s water agency says it is safe straight from the tap with no filter, and Japan and Korea treat theirs to the same standard. Rome’s nasoni fountains, Lisbon’s cafés and a London pub will all fill your bottle. Locals who buy bottled in Seoul or Lisbon do it for taste.
Where to buy or filter. Bangkok and Mexico City. Bangkok’s water leaves the plant to drinking standard and then travels through old pipes; Thais do not drink it and neither should you. In Mexico City nobody drinks the tap, and the US travel-health advice for Mexico is tap water and tap-water ice both off. Buy sealed bottles, or carry a filter bottle and refill from the hotel’s garrafón, the big jug of purified water every hotel and café has.
Ice. In those two cities, ice in restaurants and cafés comes from purified-water plants; the giveaway is a cylinder with a hole through it. Ice shaved from a block at a roadside stall is the risk. Brush your teeth from a bottle in Mexico City; it costs almost nothing.
Reading a stall. Two rules: a line of locals, and food cooked in front of you. Turnover means nothing sits in the heat, and a hot wok kills most of what is on the food, though not what is in the water. Order what the stall is known for and what is being made right now, not the tray cooked at noon. Salads, cut fruit that has been sitting and anything with raw egg are where people come unstuck; fruit peeled and bagged while you watch is fine.
What to carry. Oral rehydration sachets, a strip of loperamide, hand sanitizer, and a filter bottle for Bangkok or Mexico City. Clean hands before you eat matter more than anything on the menu.
If it happens. Most travelers’ stomach bugs burn out within a few days; the job is fluids. Mix the sachets with bottled or boiled water, sip through the day and skip sugary drinks, which make it worse. Loperamide buys you a flight or a day, but not with a fever or blood. Pharmacies in all of these cities sell what you need without a prescription, and a hotel can call a doctor.
On the day. Put the stall or market on the plan for lunch rather than the last meal before a flight, and drink more than you think you need in Bangkok and Mexico City. If one of the group is unwell, let the plan slip a day; the museums will still be open, and you can ask Waystop’s Chat to rearrange the stops and apply the change.