The above posters are assuming that you are trying to determine the horizontal center of gravity and not the vertical center of gravity, or so it appears to me. If that is so, the travel left method may work quite well or you may use a slightly more tedious method.
Hang a weighted string over the side from the deck edge. Have someone walk forward to the bow. Measure the angle that the string has moved off vertical. Reposition the string to various fore and aft locations while having the same person return to the same spot forward each time. The position where the angle is the smallest will be the longitudinal center of flotation or the center of gravity at the waterplane. This should be close enough for your purposes.
If you desire the vertical center of gravity with any degree of accuracy and you do not have the hydrostatic curves of the vessel it gets a bit more involved. I'd refer you to John LaDage's Stability and Trim for the Ship's Officer where you'll get a concise explanation of terms used as well as some practical advise. You'll need to calculate the vessel's metacentric height through a somewhat convoluted series of steps and then do an inclining experiment across the deck of the vessel with a weight. That will yield KG or the height of the CG above the keel.
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