A vessel underway, when hailed by a Coast Guard vessel, is required to heave to, or maneuver in such a manner that permits a boarding officer to come aboard.

Other federal, state and local law enforcement officials may board and examine a vessel. The Coast Guard may impose a civil penalty up to $1,000 for failure to comply with equipment requirements; failure to report a boating accident; or comply with other federal regulations. Failure to comply with the Inland Navigation Rules Act of 1980 can result in a civil penalty up to $5,000. 

Details of the Act are available online or through the U.S. Coast Guard and the Coast Guard Auxiliary:

https://www.uscg.mil/hq/cg5/cg553/NAVStandards/Nav%20Rules.asp