No, it's ok to drink from the cup if it has a sipper lid. If it's a bottle, it's also ok. If it's just water in a cup without a lid, it's ok. They just don't want any spillage. The reason for the policy is that if it were a free for all allowing all food and drinks, potentially after a few hours, buses would have to be taken out of service for cleaning. It also can become a safety issue. And you know who almost always has either a bottled drink or cup with a lid on-board the bus,........... the driver.