What term is used to describe that the towing ship and its tow are riding the crests and troughs of waves at the same time?

The term that accurately describes the towing ship and its tow riding the crests and troughs of waves at the same time is "in phase." In this context, "in phase" means that both the towing ship and the tow are synchronously moving together with the wave motion. This alignment results in both vessels experiencing the peaks (crests) and the low points (troughs) of the waves at the same time, leading to a smoother and more stable towing operation.

The other terms do not convey this specific synchronization effectively. "In sequence" suggests a following order rather than simultaneous movement. "In step" typically refers to coordinated movement, but not specifically in relation to waves. "In line" generally denotes alignment in a straight manner rather than the coordinated movement with wave dynamics. Therefore, "in phase" is the most precise term for describing the situation where both the towing ship and its tow are experiencing the wave's crests and troughs together.