It's important that the PA announcement be reflective of the actual situation so as to not mislead the passengers. It may go something like this.
Ladies and gentlemen we are having a slight problem with our instruments. They are doing some funny things and we're not sure how accurate they may be. In an abundance of caution we have decided to attempt a ditching rather than subject you to a long diversion and an unsure outcome.
Please remain calm and we will try as well