The decomposition of cedar trees depends on where they grew. Cedars from arid climates, that grow slowly, have lot of the red wood that decomposes slowly. Cedars like I have on my property have red centers, but the rest is white. To give you an idea, a 4" fence post from my cedar will last about 10 years. West Texas Cedar is supposed to last 20-25 yrs. Not sure about under water.
