Diaz wants you to get to a yacht that has some "merchandise" on it, so go down to the jetty and get in the boat. After you see the 4 boats split off do not follow them! Instead go under the 2 bridges. After you pass under the second bridge, you will see a waterway to the right that leads directly to the yacht!
After you pick up the cargo, you will have to take out the enemies while Lance drives. This shouldn't be hard, but be quick in taking them out, as the boat can't take much damage. Watch out for the helicopter that tries to take you out and take it out. Return to the Diaz Estate for a reward of $10000.