In a situation where a seaplane and a motorboat are on crossing courses, which vessel has the right-of-way if the motorboat is to the left of the seaplane?

When determining right-of-way situations on the water, the rules established by the Navigation Rules (COLREGs) apply. According to these rules, when a seaplane is in the vicinity of a motorboat, the seaplane is generally considered a "vessel constrained by its draft," which grants it the right-of-way over the motorboat, especially when the motorboat is to the left of the seaplane.

In maritime navigation, the concept of crossing courses is essential. If two vessels are on a collision course, the vessel that is to the right typically has the right-of-way. In this scenario, the motorboat being to the left of the seaplane indicates that it is the intruder in the seaplane's path. Thus, the seaplane can continue its course and speed while the motorboat must take action to avoid a collision, affirming that the seaplane holds priority in this circumstance.

To summarize, the seaplane is accorded the right-of-way due to the navigation rules that prioritize aircraft over smaller vessels on the water when their paths intersect, particularly when the motorboat is positioned to the left.