I can't catch the boat I rented, how can I cancel?

If it is within the scope of free cancellation, you can get your prepayment refunded. If it's not covered by the free cancellation, you can try to delay it by talking to the boat owner. Cancellation policies are set to protect the rights of both the boat owner and the guest. Go to the "my reservations" tab in your profile. Then enter the reservation details and press "Cancel".