Stunt Boat Challenge
This one is quite easy, nothing much to say about it. Use the boat to pass the markers and ignore the "witty" comments from Rico. The time is enough, so you don't need to go max speed at all times, just avoid bumping into the coast too much.
Cannon Fodder
The Cubans are trying to kill some Haitians but fail to do it so you need to help. Get a car with 4 doors (anything will suffice) and use it to pick up the Cubans. Drive to the factory and park in the marker. The Cubans will exit with you. Kill the Haitians behind the parked car in the alley way (if you have a M4 or M60 with you, just blow up the car). Once they are dead, a cutscene will trigger and three Cubans will run to be killed by a sniper. Take the sniper rifle or use the one you already have (if you have one) to kill the sniper. Follow the Cubans and kill the Haitians as you see them until one of them tells you to take the van. Use some tear gas (which you can find at the Police Station) to make it harder for the Haitians to carjack you. Be careful though, killing any Cubans will make you fail the mission. Use the van to drive back to Robina, some Haitians will try and follow you but that shouldn't be a problem.
This one is more complicated, be sure to have full armor and health and preferably a M60 as well, a M4 is the least you should have. Drive south to the harbor and step into the marker. Rico will drive the boat and your job is it to shoot Haitians. Use the M4 or M60 and kill as many as you can as fast as you can. Once you killed the once on the boats, Rico will drop you off near the villa. Kill any remaining Haitians, using the walls as cover (and a sniper rifle if you need to). Pick up the briefcases afterwards and you will get a four-star wanted level — not fun! Rico and the boat are destroyed in a cut scene, so run to the car parked left to the villa. If you are low on health or don't want to drive to Robina, use the car to go to your nearby estate (provided you already completed those missions) to restock, then take the chopper on the roof to fly to the drop-off. Be careful tho, cops will spawn in your estate as well(!).
Trojan Voodoo
This is a straightforward mission, all you have to do is blow up a Haitian drug factory. Start off by grabbing a Haitian Voodoo Car (which you can find near Auntie Poulet's house), then heading out to the rendezvous point with the other Cubans. Follow them to the alleyway, and stay in the car until you get to the pink marker. Get out, and take out all the Haitians, even the ones by the gate; this will make it easier to escape. After all the Haitians are dead, proceed with planting the bombs. You will only have 45 seconds to plant all 3 bombs and escape. Once all the bombs are planted, escape using the stairs next to the red building. Once you complete this mission you are done doing any more work for the Cubans, and you get a cool $10000!