I am building a small deck over a slab of cement and I was gonna use the green treated wood they have at Home Depot and Lowes. I was thinking of throwing a coat of Thompsons water seal over it to further protect it but is this a mere waste of time and money?
Thanks for the help.

I wouldn't call it a waste, it's certainly going to make a difference.

i think what most people do is wait until thier treated wood starts to show some damage, then start painting/staining it, which is already a losing battle.

While it's certainly not NEEEDED, your waterseal is by far not a waste.

Were it me , I'd add some finish/protectant soon. Having said that, I am in the process of repairing a 23 year old \PT wood fence. I took the planks off and snipped the ends and ran them through my planer to expose some very strong and not too checked wood (which I am re-using in the new fencing) The post were set in Concrete but were wabbling (I thought from rot as I can't gain access to the other side )(Witch of a neighbor) but found them sound and true. I used a Jack to lift them out of the holes and with a few wacks with a hammer the concrete popped off. I washed them down with my power washer and bleach and they look fine enough to use in another project. Only casulty in the 23 years of exposure to one side and constant shade on the other was the cross members which were warped, split and really funky. The Galvanized nails are as they were 23 yrs ago when I pounded them in. This fence was untreated with any form of finish or preservative and showed the wear. Gray and green , checked and warped but sound all the same. But to keep a fine looking project you must put some kind of preservative (PS... I don't like Thompson's)

I have built a host of decks in the past 30 years and have reccommended that you wait a week or so and then add a finish. Sun collects a heavy toll on the open decking especially in the winter. and even in the shade there is always mold and such to contend with. I's advise on selecting a good surface finish and apply every couple of years. Not paint, stain, or Semi transparent finishes as they tend to darken after each use. Stay with clear or lightly tinted finishes and replenish often. UV rays are devistating on wood surfaces.