This is a Gravina Crime Family mission.
"You got Big Sal just where you want him. Of course he's called his ass-kissers in for one last smooch. No matter, you'd have to deal with them eventually. Kill them all and finish the Gravina family for good."
It's time to finish off Sal so start the mission then drive out to the docks where you'll come to three Hank B trucks and a few gangsters blocking the way. From this point you have two ways of proceeding. One's quick and easy and the other's much harder.
The quick and easy method
Take out the gangsters there and jump in one of the Hank B's. Though they're not bulletproof, they offer really good protection and you won't lose too much health along the way. Drive through the area and go out to the end of the pier near the lighthouse where Sal is hiding. When you get out of the truck you should only have a few gangsters back here to take out; once they're dead start running towards the lighthouse.
At some point Sal should come out from behind the lighthouse and open fire on you. Kill him and start heading back the way you came. The Hank B will have disappeared so you're going to have to go back on foot. Stick to the right side of the pier and you shouldn't face too many along the way. The gangsters will respawn continually so just kill the ones who pose a threat and keep moving towards the exit until you get to the safe zone to end the mission.
The much harder method
If you're up for a challenge or you're working on your kill count for the Massacre achievement, you can make your way to Sal on foot. Keep in mind that they'll respawn continually and often from behind, so don't stay in any one place for long, as you can easily find yourself surrounded and out of ammo very quickly.
Once you're done here get your health and ammo fully replenished in preparation for the final mission.