Once you've been caught trying to sneak a whole bunch of soldiers into their harbor covertly, nobody's going to trust any other supposedly innocent ships to be innocent.
So what you are saying is that any ship Lincoln had sent, be it a full-blown ship-of-the-line or a 10 foot row boat, would have been fired upon since Davis didn't trust anyone. So if the supply ship was going to be fired upon no matter what then put the supplies in a ship that could defend itself from Confederate aggression.