What licensing is required when operating an amateur station aboard a US-registered vessel in international waters?

When operating aboard a US-registered vessel in international waters, FCC Part 97 rules still apply to your amateur station. Therefore, any FCC-issued amateur license allows you to operate, provided you adhere strictly to the frequency privileges and modes authorized by your specific license class. There is no such thing as an 'FCC Marine or Aircraft endorsement' for amateur radio licenses. Furthermore, you do not need a commercial Radiotelephone Operator Permit to operate an amateur station, nor are operations restricted only to General class licensees or higher.