The cache is not at the posted coordinates, you might get a bit wet if you go there!!
You need to complete the Bridges of Barnstaple Adventure Lab first, where you will be given the coordinates for the bonus cache and the hint.
Would be great to see some photos of your walk, although this is not a requirement.
Enjoy the walk 
Please rehide well.
You will need to download the Adventure Lab App from Groundspeak.