When must a WIG craft keep well clear of all other vessels?

A WIG (Wing-in-Ground) craft must keep well clear of all other vessels during takeoff and landing because these phases of operation involve specific flight dynamics and low altitudes that can make the craft less maneuverable and more susceptible to disturbances from other vessels. During takeoff and landing, a WIG craft is particularly vulnerable and requires a clear operational space to safely ascend or descend without risking a collision. Maintaining a safe distance from other vessels is crucial to avoiding potential hazards during these critical times.

In contrast, the necessity for keeping clear is not primarily dictated by bad weather, being moored, or the craft being powered off. While those scenarios may present certain considerations, they do not involve the inherent flight characteristics and operational risks that are present during takeoff and landing of a WIG craft. Therefore, option A is the most relevant and accurate context for when a WIG craft needs to maintain distance from other vessels.