This is a Wherigo cache, which can only be found by downloading a Cartridge on to your GPSr or using an app on your phone. I have tested it with the Wherigo app on my phone using iOS and it worked fine, and the Cartridge itself has been used for numerous caches around the world. You will need to download the Reverse Wherigo Cartridge created by -Waldmeister-, which is available to download here.
The Cartridge tells you how far away you are from the final cache hide, but not the direction in which you need to go. Once you start the Cartridge going, set it up by choosing your languague and measurement unit. Then input that you want to Play (not Create) a Reverse Wherigo. Then input three sets of 6 numbers (see below) one at a time. You can then look in your Inventory for the Reverse Cache Box. Click on that to display the distance to the cache. The Cartridge keeps a tally of how many times you look at the Reverse Cache Box to get the distance, although it doesn't seem to matter if you look numerous times (as far as I have experienced).
Start at the posted coordinates: a large set of steps giving access to the River Wye. There is Pay & Display (seasonally) parking very nearby, but to access it you need to drive under the A40 from near Monmouth Leisure Centre. Walking in from free parking elsewhere in town might be easier. At the given coordinates you will find a plaque and a carved date. The date on the plaque is ABCD and the carved date is EFGH. Use those numbers to make the three lines of 6 digits for the Wherigo Cartridge.
A D 5 G 3 2
B F 4 7 C 5
D 4 F E 0 A
Keep hold of these numbers: if the cartridge crashes you will need to put them in again to continue. Once you complete the cartridge you will get a code so that you can create your own Reverse Wherigo anywhere you like, just make sure you tell Waldmeister about your cache.
Please don't say anything in your log that will give the direction or distance away. No boat or swimming is required. Have fun!