Water main and Water pipe

Water main noun - Main (a pipe or conduit) for conveying water.

Water pipe and water main are semantically related in waterway topic. In some cases you can use "Water pipe" instead a noun phrase "Water main", when it comes to topics like conduit, waterline.

Water pipe noun - An oriental tobacco pipe with a long flexible tube connected to a container where the smoke is cooled by passing through water.

Water main and water pipe are semantically related in waterway topic. You can use "Water main" instead a noun phrase "Water pipe", if it concerns topics such as conduit, waterline, pipe through which water moves.