The instructions were to peacefully reprovision the fort. If the ships were opposed and forces on the ground did not allow peaceful passage of the supply ships there was an authorization to use force. That left a lot of discretion to those on the ground. The orders indicate that the intent was peaceful and that the fleet was not sent to trick or provoke the rebels to start a war.
There was an instruction to use force. It wasn't a suggestion.
"Should the authorities at Charleston, however, refuse to permit or attempt to prevent the vessel or vessels having supplies on board from entering the harbor, or from peaceably proceeding to Fort Sumter, you will protect the transports or boats of the expedition in the object of their mission-disposing of your force in such manner as to open the way for their ingress and afford, so far as practicable, security to the men and boats, and repelling by force, if necessary, all obstructions towards provisioning the fort and re-enforcing it; for in case of resistance to the peaceable primary object of the expedition a re-enforcement of the garrison will also be attempted. "
The Statement from Simon Cameron was more direct.
" WAR DEPARTMENT, Washington, April 4, 1861.Captain G. V. FOX, Washington, D. C.:
SIR: It having been decided to succor Fort Sumter you have been selected for this important duty. Accordingly you will take charge of the transports in New York having the troops and supplies on board to the entrance of Charleston Harbor, and endeavor, in the first instance, to deliver the subsistence. If you are opposed in this you are directed to report the fact to the senior naval officer of the harbor, who will be instructed by the Secretary of the Navy to use his entire force to open a passage, when you will, if possible, effect an entrance and place both troops and supplies in Fort Sumter.
I am, sir, very respectfully, your obedient servant,
SIMON CAMERON,
Secretary of War.
For that mater, David Porter said in his memoirs that if those ships had tried to do something, they would have all been sunk.