Unless a lot behind the scenes has changed in the last few yearsI havent kept up on.....
Maybe the first time they ask.for permisdion, then anything in plain sight of a safety inspection they find is fair game...
if you say no, then tbey say "heave to and prepare to be boarded"....
and at that point, anything found in plain sght during their normal safety boarding is still fair game.
I will say, if there is probable cause that you are doing something illegal, as I was involved with countless times, the USCG will request a warrant and shadow you toll they get it. Then they will board.
But for a routine boarding, a no to their boarding request might get you a wave and goodbye, or it may oromot a warrant search where they bring chainsaws, dogs and interagency friends aboard along with big guns.