The maximum waterline beam (BWL) is the widest point of the hull where it meets the waterline. This is usually different from the beam quoted in the vessel specs, eg moulded beam. The effect of BWL on gyro sizing is further discussed in the What is the GMt? whitepaper available for download on the VEEM Gyro website..
The Displacement, BWL and GMt at various loading conditions can be found in the stability book, located on the bridge.