I don't think so if I only come to the aid of vessels in distress. Now, if I went on the offensive, then I would need a letter of marque.
Letter of Marque and Reprisal = ability to take enemy vessels as prizes during war. Traditionally issued to all naval vessels in time of war. Must specify who the enemy is.
It is the duty of all military vessels to recapture/liberate vessels captured by pirates. If the recapture occurs 24 hours after the capture, the salvage goes to the liberating vessel’s crew. Unless someone in congress changed the rules.
Of course, I could be wrong.