Unfortunately, this isn't terribly different from the experiences some of my friends have had with US Customs, when returning from sailing trips in the Caribbean. Searches that actually damage boat fittings have occurred; upholstery opened up, etc.
Some agents seem to presume that if you own a large boat, you must be smuggling drugs or weapons.
I have not yet heard of a successful suit being brought against Customs for such actions - though again, this was US Customs, whereas the OP's parents seemed to have had their issues with Canadian Customs.
I just would not be surprised if they have similar (apparent lack of) rules to what our guys seem to have.