Ah bon, c'est tout ?

Follow the road to 13888 7317 and jump in to the water

OK, you want to start at the road that leads up to the Stormhold fortress. Just before you enter the keep itself, you will see a blue lightning covered sail thingy on the right side of the gate. Take the path to the RIGHT of that gate, and you'll come to a cliff that overlooks a cove with a big ribcage in it. you want to jump off at aiming for the two biggest ribs in the ribcage, but DON'T JUMP YET!! You have to be in The Black Strand, not Stormhold, so make sure you edge to the right of the cliff, just to the just the the tippy pointy bits get a bit darker, just before you fall off the map will change to The Black Strand and you're at the right place. If I ever figure out how to insert screenshots I'll put some in to show exactly where to stand. Now, mount up, because otherwise you'll hit the rocks at the bottom and you can't get a run up to jump in. It seems to be really touchy about where your feet leave the ground at the top. 12645:5750 is where I stood to get my two step and jump into the water.