It doesn't make any sense that a semi-transparent stain would not
penetrate. Perhaps it does have something to do with the water under
the deck....

btw, why is there standing water under the deck? I have to think it's
effecting at least the underside. If you plan to leave the water there,
I'd suggest pulling the the parts you can and treating them after
they've completely dried.

Are you sure they cleaned it properly and allowed it to dry completely?
Maybe they rushed things. I'm thinking you lucked out that both coats
are coming off so easily. At least that way you can start, basically,
from scratch.

As others have mentioned, you need to find out what exactly they used
to seal the deck. I'd also try to find out what they used to clean it.
With that information -- and I think you'd have to tell them about the
water underneath -- I'd talk to a paint store [I'd go to 2 because some
stores have better knowledge than others]. I'd also suggest talking to
someone at a lumber yard, and possibly, if there's one in your area,
someone at a woodworking store. Just stay away from the home centers.

The following website reccomends a water-repellent preservative a
little above a semi-transparent oil-based stain...

http://www.wrcla.org/finishingcedar/...s/overview.asp

You'd have to read the labels, but I don't think 68F is a bad temp for
application.
Let us know what you find out -- cedar decks are beautiful and uncommon.