Bill Johnson was a US Marine veteran and an excellent sniper. Initially, he was unwilling to work with The Organization. When he was under Carla's orders, he was given a mission to execute a unnamed target on a ferry boat.
The Dragunov sniper rifle that Michael retrieved with Victor is given to Johnson, so he can accomplish his mission.
We never know who the target of Bill Johnson is because he is killed by a booby trap set up by Victor.