The correct answer is C) Accidental stranding or grounding. When underway with a tow, the Coast Guard must be notified in the event of an accidental stranding or grounding. This is required by Coast Guard regulations, as an accidental stranding or grounding could pose a significant safety and environmental risk. The other answer choices, while potentially reportable under different circumstances, do not specifically require notification to the Coast Guard when underway with a tow. For example, an injury requiring first aid (A) or damage to property over a certain dollar amount (B) may need to be reported, but are not the same as the mandatory notification required for an accidental stranding or grounding (C). Loss of bridge-to-bridge radio capability (D) is also not a specific requirement for notification to the Coast Guard when underway with a tow.