Ive heard some horror stories of coast gaurds just up and boarsing peoples boats with no probable cause and just plain being jerks.
My guess is that you were being told some very exaggerated stories. We've been boarded twice by the USCG--- yes, they can do it without probable cause or warrants--- and they were no hassle at all. The boardings are conducted as safety inspections, and they do that, checking for fire extinguishers, PFDs, and all the other stuff a boat is required to have. But the real reason is to add an element of risk to people engaged in drug trafficking, human trafficking, and so on. Since the boardings are random there's no telling which boat they may choose to board. And I'm sure they have a whole range of characteristics they are on the lookout for, as well.
Their inspections were conducted courteously, they were not intrusive, and they did not impede our cruise. We simply kept going while the inspection was being conducted. I know other boaters in our area who have been boarded and in all cases their experiences were the same.
Sunchaser made a good point. If you have problems with authority organizations boating may not be the best pasttime for you. You will most likely be dealing with the USCG at some point, and there can be harbor police depending on where you're boating. States imposes taxes and registration fees and may have enforceable requirements for displaying numbers or decals. Around here the county sheriff's departments walk the marina docks periodically checking for expired or missing annual registration stickers. You get fined if you don't have a current one displayed,
If you're in a marina, particularly a city, county, or state marina, there will be all sorts of "you can do this and not that" rules. And of course there are the wonderful folks in the insurance industry who can and will impose requirements on you to fulfill their policy requirements.
There's a hell of a lot more to owning a boat than just buying one and driving it around.
