Sadly with good prizes on offer, there is a need to worry about cheating.
My advice would be to decide well in advance what the rules are, publish them in advance and make sure all competitors get a copy on registration.
This also includes the consequences of breaking the rules, and who is responsible for govering these rules and other things like species identification.
My thoughts would be that anyone caught breaking rules is disqualified.
At the yacht club with boats that all look the same we sometimes issue stick on large bow numbers, these have to be attached prior to the comp starting and stay on the boats until after the prizegiving. In this case it may also be worth taking photos of the kayak and crew in case of identification disputes.
Good luck with the comp, if you need more assistance we may be a available to assist