What's with Sal and his payphones?
You obtain another payphone call from Sal. Someone has found out the Sicilians were behind the clash of the Sindaccos and Forellis that almost put Sal out of his misery, and that someone is going to tell Franco Forelli all about it. It would be an advantage if this message didn't get to him...
The messenger is in a speedboat on LC waterways. Grab the boat at Bedford Point, reach the messenger and drive-by his boat until he's dead. Simple.
It helps if you have an MP5 or any other Sub-Machine Gun so you can drop the messenger.