If for some reason the watercourse isn’t included on an insurance application, it can lead to problems come renewal time. Whether it’s intentional or genuine error, if you fail to mention this vital detail on your application, the insurer will deem it to be non-disclosure.
The best possible outcome is that the insurer continues to cover your home, but with an increase in premium. The worst case scenario: the insurer refuses to renew the policy (they can research whether you have been declined by other insurers), resulting in you having to buy non-standard policies. This can continue into the future, potentially costing you more in the long term.
Best practice is to simply be very careful (and honest) when completing an application. And read all the information in the quote to make sure the details are accurate.