Basically, the decocker safely lowers the hammer (usually with the firing pin blocked) to a half-cocked position when pressed. You can then carry with the hammer in the half-cocked position (lighter pull than full DA, but harder pull than SA). If it's decock only, then most people would carry it in that way.
If you're dead set on carrying cocked and locked, then you need a manual safety. Some guns have one lever that acts BOTH as a manual safety and decocker (e.g. the H&K USP variant 1). The USP Variant 1 has a lever that flips up for manual safety, down to fire, and further down to decock. So with that, you get the best of all worlds.
Personally, I have a CZ 75 BD (D = decocker only). I have fired guns with manual safeties, but I don't see what all the hype about cocked & locked carry is about. I find the manual safety easier to accidentally flip off than a half-cocked trigger is to pull, so I think it's safer to carry a decocker model. Plus, I don't find the half-cocked pull to be all that bad, and I'm still accurate with it, so I have no reason to carry C&L. Furthermore, I would rather not have to fiddle with a safety if the SHTF--I prefer point and shoot. I don't like anything extra to have to be done before I can pull the trigger.
Do decock-only pistols still fire in DA/SA?
Yes. After the first pull, all subsequent pulls are SA.