I used to fill my Smoke Vault-24 gasser pan about 1/2 full of pea-gravel...worked very well. I tried 4lbs in my WSM-18, but it seemed like it might have been too much...sluggish response when adjustments were made, but then it did hold pretty steady temps.
After a couple months I opted to run dry with just a foil liner in the pan and the WSM still runs pretty steady temps.
If the mass of your smoker is a decent amount you may not see much benefit from added thermal mass in the water pan. I am an advocate of using little to no water, though, but with a foil liner in the pan for drippings, so they don't scorch and make your meat taste like it's roasted.