A lot of ill-informed people will confuse "Legal" with "Company Policy". I had a waitress at a restaurant tell me it was a City Law that they had to card everyone during the World Cup, and that the City did not allow out of state licenses. This would require the city actually pass such a law. What they meant was "Company Policy", which is fine, but not the same thing.
I would ask them if the City, County, State or Federal Government passed the law restricting price negotiations on H20 boats. I would further ask him to bring his sales manager in to show me where that law was published. ALL Laws are published and accessible by the people.
More than likely, there is not sufficient wiggle-room on an entry level boat, or the salesperson is using an odd negotiation tactic, hoping that will shut you down.