There’s no standard collective noun that can be used for a group of kayakers. Depending on the context, many different nouns may serve as collective nouns. Ask kayakers for their take on this, and you’re in for a laugh. It’s called a “wobble of kayakers” if the group consists of beginners – or a “swell of kayakers” if they’re advanced paddlers. “Flotilla” and “YakPack” are hilarious, too. But you can always go with something simple, like “fleet.”