When you head out on a group whitewater rafting trip, the outfitter will have PFDs for you. Commercial rafters typically wear rafting-specific type V PFDs. These safety jackets are designed for optimal safety and comfort for a day spent river rafting.
Some outfitters have different PFDs depending on what trip they're taking you on. It's safety, not comfort, that's paramount - although most life vests are pretty comfortable.
Conversely, you may not need a heavy-duty jacket on a flat water family rafting trip. When you're floating down calm stretches of river, there's not as much concern about going overboard - although you always should be prepared for just that situation. If something were to happen, it's much easier to get a rafter back into the raft safely in calm water than in churning waves.
To find the right vest, you must first select the right age group. There are child and infant vests, youth sizes, and adult life vests. There are then different sizes of adult life jackets designed for various body shapes and sizes. The best PFD: is comfortably snug around you torso, doesn't ride up too far when you raise your arms overhead, stays put if someone gently tugs the shoulder straps upwards, permits comfortable, unrestricted movement.