Short answer: no deathwatch in
40k.
Long answer:
GW have forgotten about them. The
GK codex appears to be "THE INQUISITION" codex badly misnamed. The
SoB codex will, I expect, be "THE ECHLIASTY" rather less badly misnamed.
Xenos & Deathwatch, being akin to Malleus & Grey Knights simpley have nowhere to be within the codex structure.
If you want to represent Deathwatch, your best bet is with Codex: Space Marines. Deathwatch fluff being that the 'best of the best' of the space marine chapters are seconded to the Deathwatch (which I think makes them the 'best of the best of the best'). Deathwatch has no scouts & makes use of heavily upgraded bolt technology, as well as the standard special weaponary of the Space Marines. They also use heavy weapons (and had a special rule for 'suspensors' which demished range, but allowed them to be fired on the move, but there aren't current rules for that).
So with that in mind, Kantor & Sternguard armies would probably best represent them. If you want inqisition elements, then a librarian (as an =][=) and scrunts (as =][= stormtroopers) could work at a pinch, or, for a couple of months, you could use Codex Witchhunters.
As to how they operate, there is a Deathwatch novel from Black Library (IIRC), but there isn't a great deal within codex etc. sources (unless you google for the Deathwatch 3rd/4th ed special rules, which has some background etc). So any type of army (mech, foot, drop pod, etc) is possible. Dreads are a source of contention (as they don't have their own marines, how would they inter anyone), but I wouldn't get too hung up about it.
All the above is from memory, so anyone feel free to slap me down!
Hope that helps.