I think you should tie it down with weights. What if you want to move it later or redo the tank? Do you really want it glued down even though it would eventually sink on its own anyway?
Whatever you do dont use super glue. Aquarium silicone is cheap and works like a charm. Iv had the same driftwood in my tank for years and it still floats. I just wedge it between 2 rocks. Every few weeks Iv gotta stick it it again, but it doesnt bother me.