Old Marcus is located near Marcus, WA. The original town was one of many that was scrapped and moved when the Columbia River was flooded to create Lake Roosevelt. While there are no structures left there are many old foundations and such still spread out in the water.
It appears you can access if from shore by parking in the pull out just south of town. We did the dive from a boat though as it would be quite a surface swim.
I did not bring my UW camera on this trip but I was able to take some from the boat to help mark the location. Next time we go I will bring the GPS and get good numbers.
If you look at the NOAA Navigation charts I have linked on the previous page you can see the depth is generally 20'-30'. Of course when the water levels are let down many of these bases are exposed.
From the center of there river looking toward HWY 25 you can see the pull out in the top center of the pic.
Swim straight out. When you look North and are close to parallel with the end of the island descend.