General Discussion

They include offshore islands, sounds, bays, rivers, and creeks to the head of tidewater or to a point where tidal waters narrow to a width of 100 feet. Most people would recognize the coastline as the outer edge of the states along the bodies of water (Atlantic Ocean, Pacific Ocean, Gulf of Mexico). The driving distance from Key West to Pensacola is only 932 miles, so it doesn't make a lot of sense when doing the comparison to consider Florida's Gulf Coast as over 5000 miles.

https://www.google.com/maps/vt/data=3Y8oYzzAfIVWLEwmYM7aqGelP8DPojYfVQTh7IZD_AStplvKt3DCk_ab6e2Fe1rgN79jUSpNpJM8kwViobxyp2GFAjFow-auu5PcLD8E8G1I4h-mxr0kJavV2vu_eU9xesFl5AIljLEcrYYZlNewgmtwsyVgoZgpaFw-l7Cr7b5cPmk-wLH3,eO0lY1zIhY40EgkGN_lXsyCAQGtFTsfc38xPx3gbPTlFLUsPB1VJ7y7RrNdtI3eqFET8nhniWLCqNZJITI7yWpJtNlP66NPChO42xYypdm-lGuWpw6JBam6ZUj7jahLFDrmerIX-YAEGzcRU0nKKSRrUwPjssxwIfkmhJuE-75iettuI5p1mecTApTGLtR4tBDUQcDnuzOIkk1fAK2fkSWdo3UB7i6BDKi22H3uK1_vRvGS-qCkOLJMGSQKNq4Tjz4zBdlNKQhc18PP9FO5OdB2tJe_NMJgh_gvjZOKEK3Lk-Kwe1gemb6VYLLTjmiIXvVCAGE8j-qTbj91UOMkQRIYhPAQyOXXbYwQCdbH8ZzJDoI86T94cxpQWO4xcImplUOm71AiZNMV8ZlwVnqjzx7iPCTUFzg

Using the logical definition, the US portion of the coastline spans 2,700 km (1,700 miles). The Mexican coastline spans 2,805 km (1,743 miles). Mexico's portion is indeed longer.