Via PogoWasRight a link to Coast Guard Boardings and Your Fourth Amendment Rights, Part 1. According to the author, the 4th Amendment has no traction at sea: the Coast Guard can board US flag ships at will, whether on the seas, on rivers, or in port, without the least suspicion.
Sorry, but when it comes to Coast Guard boardings, you don’t have any rights.
I’m surprised how many boaters don’t know this. The US Coast Guard can board your boat any time they want, and look anywhere they want, without probable cause or a warrant. They can do this on the open sea, or while you’re asleep aboard in your marina at midnight. They can look through your bedsheets, in your lockers, in your bilges, in your jewelry box, or in your pockets. They can do it carrying just their sidearms, or they can do it carrying assault rifles. They can be polite about it or they can be rude, but mostly they’re polite.
The article does not say, but I presume this even applies to houseboats?