Have you looked at maps of the area? OKW and SSR are on the same road. The entrance to OKW is first, then the treehouses then the main SSR. Basically drive out to the road, make a left, another left almost immediately and you're at the OKW guard house. A taxi would take you 5 minutes, the boat at least an hour and like you said, chance of missing it.
If you don't have a car, take a taxi.