In what situation should vessels take particular caution?

Vessels should take particular caution when constrained by their draught because this situation significantly affects their maneuverability and operational limits. When a vessel's draught is close to the depth of the water, it can become vulnerable to grounding, making it challenging to navigate safely. In such conditions, the vessel may not be able to turn or stop as quickly as desired, and nearby navigational hazards may pose a significant risk. Moreover, vessels constrained by draught need to be more vigilant about their surroundings and the actions of other vessels to avoid collisions or accidents.

In contrast, the other situations presented do not inherently require heightened caution relative to navigation and safety standards. For example, when there are no other vessels around, there is often less risk of collision, and while racing may demand attention, it doesn't apply to all vessels under normal operations. Shallow waters alone can require caution, but it's the draught constraint that is a critical factor affecting a vessel’s navigation capabilities in various scenarios.