One feature of a watertube boiler is that:

A watertube boiler operates by circulating water through tubes, which is a defining characteristic of this type of boiler. In a watertube boiler, water passes through the tubes that are heated externally by combustion gases. This design allows for efficient heat transfer and provides a significant advantage in terms of steam generation and operational flexibility, especially at higher pressures.

In contrast, other features listed do not accurately describe watertube boilers. While firetube boilers typically rely on gravity for water flow, watertube boilers can operate with pumps to circulate water, which enhances their efficiency and operational capabilities. Furthermore, watertube boilers often have a larger heating surface area due to the arrangement of tubes and the ability to accommodate more tubes within the boiler structure. They also can operate effectively at higher pressures, making them suitable for various industrial applications where high-temperature steam is required.