The cache is not in the middle of the river. To find the coordinates you need to complete the Bridges of Kendal Adventure Lab which takes you on a riverside tour. You will find the cache at N 54 18.AAB, W 2 44.CDE. ABCD and E will be revealed as you complete the Adventure.
If you don't want to walk to the bonus cache, you will be able to find parking not too far away. Admire the view and watch out for hungry ducks.