Rush on over to the highlighted dock, get in the boat and chase after her, being careful to avoid the barrels and don't shoot at the boat. After a while, the papers will start blowing away into the water. You just have to run over the red marks to pick them up. You have to get them all, so if you miss one, get back to it double quick and keep chasing. Don't let her get farther than 500m away, or you'll lose her and fail the mission.
Once you reach more open water, she'll bring in some men. Just shoot back; you get unlimited ammo in boats, so you can miss as often as you want. Once you've got all the papers, destroy the boat.
You can now buy this front for $75,000. If you have Montana Holdings it will cost you $68,500.