Does anyone really expect the police or indeed anyone other than boaters to understand our etiquette of not going onto a boat without being asked?
The police went on the front deck (I understand) not in the boat, so the same as walking up the path to a front door to knock and ask about a missing person.
There is a thing called implied right od access, I never consented to anyone entering my property (path/garden) to knock on the door but they do or to put letters and leaflets through it, unless there is a sign to the contrary then anyone can do so, in this case the police lady did what she thought was the right thing and if boaters do it differently then there you go. The big picture is they are looking for a vulnerable adult who may have come to harm, far more important than; should you knock from the outside or any feeling of having had your privacy infringed.
K