The weight capacity of a kayak highly depends on the type of unit. There are several types of kayaks -- recreational, expedition or touring, sit-on-top and tandem. Each type of kayak has a different weight capacity suited for the purpose that it's built. Understanding the purpose of each type of kayak will make sense of its weight capacity.
Recreational kayaks are designed for slow-moving rivers and lakes. They provide good initial stability for casual paddlers, anglers and photographers, but handle poorly in open water, windy and turbulent conditions. Recreational kayaks are designed for day touring and have a large bulkhead that provides a decent amount of storage for additional gear like a camera equipment and food. Most kayaks of this type are 12 to 15 feet long with a large cockpit area for easy entry and exit. Typically, recreational boats have a weight capacity ranging from 250 lbs. to 300 lbs. Some recreational boats designed for anglers who hull in big catches, like the Wilderness Systems Ride 135, have a weight capacity of 400 lbs.
Expedition or Touring Kayaks
Expedition or touring kayaks are designed for multiday adventures. As a result, these boats tend to be much longer, ranging 12 to 16 feet, and have a large storage capacity area for extensive food rations and expedition gear, like tent, cooking equipment and sleeping bags. They are designed to travel long distance and are incredibly stable in turbulent, windy and open-water conditions, which is why they're also called sea kayaks. To make room for all the extra gear people take with them on multiday trips, most touring kayaks have the large weight capacity of 350 lbs.
Sit-on-top kayaks are the epitome of "lazy man's boat." They're designed for beginners and children. They're built with a sealed hull with a molded cockpit or seating area. As a result, they're easy boats to get into or out of. The design gives them added stability in calm waters. The added stability makes them a great diving platform for swimming. The sit-on-top boat is considerably slower than the other boats because of its wider frame. They typically range from 10 to 15 feet in length and have a weight capacity of 350 to 400 lbs.
Tandem kayaks are designed for two-person paddling. These boats are mostly designed for recreational purposes. Typically, tandem boats are a sit-on-top style to make it easier for both people to mount. To accommodate two people, tandem boats are usually wide framed, commonly ranging from 14 to 18 feet in length. Like other sit-on-tops, the wide frame makes the boat very stable. Tandem boats have a larger weight capacity, typically ranging from 500 to 600 lbs.