Clifton Craft, Jack Ferguson, and William Wilson (plaintiffs) excavated objects from sunken ships located within the Channel Islands National Marine Sanctuary (sanctuary). The excavation involved hammering and chiseling activities that left scars on the seabed. A National Oceanic and Atmospheric Administration (NOAA) regulation (regulation), 15 C.F.R. § 935.7(a)(2)(iii), provided that no person could “dredge or otherwise alter the seabed in any way,” other than to anchor vessels or to bottom trawl from a commercial-fishing vessel. NOAA assessed civil penalties against the plaintiffs for violating the regulation. The district court affirmed the assessment. The plaintiffs appealed, claiming that the regulation was overly broad and unconstitutionally vague.