While most people will carry one or perhaps two small boats on their roof rack, some people get a little bit ambitious:
There is not only a rack on that car but 6 - count 'em - 6 boats on top of that car. Even little plastic whitewater boats like that weigh about 35 pounds each. So, we're talking about 210 pounds and another 15-20 pounds of rack for a total of at least 225 pounds. First, there are limits that the car manufacturer puts on how much weight you can put on your vehicles roof - usually somewhere around 150 pounds. Then, if you're using a commercial rack, there's another limitation on capacity for the rack, too. You shouldn't be exceeding the weight limit for either one. If you are, perhaps it's time to look at a canoe or kayak trailer to tow behind your vehicle, or have a friend help you haul boats on their vehicle with you. There's a few dangers here - one is damaging your vehicle, the other is overloading your rack and having the rack fail again potentially damaging your vehicle or having your rack and boats fall off the vehicle.
While I don't have a picture, here's another potential problem. There is a car in town that has a beautiful Wee Lassie canoe made by Laughing Loon on a nice roof rack. The issue is the canoe is mounted on the rack right-side-up. While generally this isn't an issue what happens when it rains? Yep - the canoe fills up with water. Water is heavy - 62.4 pounds per cubic foot. A bit of rain when the canoe is like this on the car can quickly overwhelm the weight capacity of the car, and rack. Also if the boat isn't strong enough, you could damage the hull of the canoe, too. Just a hint - a siphon is great for getting water out of a boat on your roof - reaching up to tip a water laden boat is something you don't need to do. It's much simpler to carry the canoe upside-down.
Here's a similar situation - but a kayak:
Generally, this set-up is good. there's a rack on the car roof, there's only one boat on the roof and it's properly oriented. Notice the kayak's cockpit - it's open. Like the canoe, it can collect water - albeit more slowly than a canoe. A cockpit cover is a cheap and easy way to keep the inside dry. Another benefit of the kayak cover is that it can help reduce drag and road noise.
Pay attention to the details. Avoid overloading!