In this mission you get to play as the assassin, which is odd as it isn't unlocked yet. Follow the red markers and get to the vantage point within the time limit. Be careful, as at the end of the run you'll have to do a series of jumps. Make sure to keep your speed up and hit the jumps at the right points to land safely at the next point. Needless to say, if you miss you've effectively failed the mission.
Now you have to scan the many boats using the sniper rifle. To enter zoom mode, press and then use and to zoom in and out. Look out for the guy wearing a blue hat and a purple jacket in a small orange boat. Don't kill random people, as that will cause everybody to speed up and make it harder. Just keep your cool.
You can now buy this store for $80,000. If you have Montana Holdings it will cost you $72,000.