33 C.F.R. § 64.06
As used in this part:
Hazard to navigation means an obstruction, usually sunken, that presents sufficient danger to navigation so as to require expeditious, affirmative action such as marking, removal, or redefinition of a designated waterway to provide for navigational safety.
High seas means those waters described in § 2.32(c) of this chapter.
Markings means the lights and other signals placed on or near structures, sunken vessels, and other obstructions for the protection of navigation.
Navigable waters of the United States means those waters described in § 2.36(a) of this chapter, specifically including the waters described in § 2.22(a)(2) of this chapter.
Obstruction means anything that restricts, endangers, or interferes with navigation.
Structures means any fixed or floating obstruction, intentionally placed in the water, which may interfere with or restrict marine navigation.
33 C.F.R. §64.06