The Coast Guard in some ways has much broader authority than state and local law enforcement. They are both a federal military agency and a federal law enforcement agency. Coast guard officers, petty officers, etc. are also designated at U.S. customs officers. In contrast to state and local law enforcement limitations the Coast Guard does not need probable cause to stop, search, detain,etc. most any vessel. Their authority comes from the U.S. federal code, specifically: -- 14 U.S.C. 89 which authorizes the Coast Guard to go onboard any vessel subject to the jurisdiction or operation of any law of the United States, whether on the high seas, or on waters over which the United States has jurisdiction, in order to make inquiries, examinations, inspections, searches, seizures, and arrests for the prevention, detection, and suppression of violations of laws of the U.S. So...they don't need reasonable suspicion or probable cause to contact a vessel. Personally I have no problem with it and am glad they're out there.
Note that this applies to any vessel subject to U.S. jurisdiction and on the high seas as well as U.S. waters so foreign vessels are fair game as are all of us.