As @Ontarkie attested, this is not enforced nor is enforcement constitutional as per courts.
If one is traveling by third party commercial carrier, such as ferry, bus, train, with at least one exception (see below) the carrier is required to deny boarding without a WHTI travel document.
Regardless each day, thousands of U.S. citizens re-enter the U.S. on cruise ships without WHTI documents, as the cruise industry and CBP have negotiated an exception to the carrier rule if the ship returns to the original port of departure.