U.S. law requires a U.S. citizen to fly to the U.S. on a U.S. passport. It does not require a U.S. citizen to walk, drive, swim, or float into the U.S. on a U.S. passport. Indeed, CBP fully embraces U.S. citizens traveling without U.S. passports on cruises, provided the same port is used departure and return.
https://www.visajourney.com/terms-of-service/
“By way of example, and not as a limitation, you agree that when using the Service, you will not:
[..]
Condone or instruct, either directly or indirectly, others on how to commit fraudulent or illegal immigration activities in any way, shape, manner or method“