You can find the remains near the bridge situated at Ivarstead. The body is in the middle of the river UNDER water.
Easiest is to deactivate all other quests in your journal and activate this one. That way it places a marker on your compass and also an icon on the remains itself as you get closer.
If you are unable to 'activate' this quest in your journal (as I had) i suggest saving up & restarting Skyrim as the remains were not there when I could not activate the quest in the Journal.
If you are on a console, turn off the console, turn it back on and start up Skyrim again with your save game. You should now be able to activate the quest and follow the marker as well as find the body itself.