The Canadians were traveling in mufti and registered as civilians so as not to violate U.S. neutrality. New York port officials were sympathetic to Britain and did not pry into details about passengers or cargo.
Almost all large passenger ships carried deck guns and the captain of the U-Boat had to assume the Lusitania did. Simpson, a Brit, maintains that the Lusitania did.
Clearly, the Lusitania was a legimate target.