There are many restrictions on what you can take through customs these days, least of which is liquids. Doesn’t matter whether it looks as if the seal is unbroken, a bottle of liquid is a big ‘no no’!
Which means planning ahead before you go through those magic doors of Immigration and Customs.
100ml is the limit which means you can’t even take a standard filled water bottle with you. I’ve found the best thing to do is empty you bottle before you go through customs, and once passed through security and airside, fill from one of the many water fountains.
This not only saves you paying an inflated price for water in a disposable bottle, but you’re also not buying a disposable bottle!. Throughout your trip, you can refill anywhere that it’s safe to do so.
Same goes for a reusable coffee cup.