I guess the first question would be what is the water clarity like. I have to fish muddy or stained water most of the year (no joke, I fish the Ohio River creeks) and I can catch fish close to the boat. But if I go to a lake with anything better than stained, I spook them so I have to stay back and make long casts (while trolling a long way in.)
Now, about nailing down that waypoint. Use your GPS, by keeping it in one location on your boat, zig-zag back and forth until you find the structure. Keeping in mind that his GPS may not be as good (or yours) so you may have to look around the location a bit.
Once you found the location of the structure, mark it, and go fish somewhere else for some hours (considering the water is clear.) So when you come back (that day or some other day) you will know it's exact location.
Josh