This works best if you've got a longship, but in shallow water a karve will work too. Position the boat so the raised bow section is right next to item in the water. You can hop up onto the bow. That will give you a little more altitude on your jump. Be sure to remove all movement-penalty armor first. Jump as high as you can and aim for the item in the water. Takes a little practice to land in the right spot but it's worked quite well for me.