Whether or not there will be Primaris Deathwatch marines “officially” in the fluff is something that remains to be seen. Originally, in the 8th Edition Index, there were no options for Primaris Deathwatch. You can’t take intercessors, inceptors, Hellblasters or any of the starter set characters as Deathwatch.
The cynic in me says that this is just another one of GW’s “no model no rules” things, whereby because the starter kits are not designed to be able to easily change the shoulder pads, whereas the new multi-part kits will be, that DW will be able to use the new stuff only.
This does seem to be the case as you can take the Reivers, Redemptor, new captain and new librarian. This presumably means we’ll get the apothecary and chaplain as well. I’d love a deathwatch apothecary, so looking forwards to this.
Fluff wise, I think I’ll probably use primaris characters and dreads, maybe the more specialised squads like agressors, but not the main line infantry squads, for my Deathwatch. I don’t see why they’d have these mass produced dudes, and I also don’t think that any chapters will send off their newly acquired primaris to the Deathwatch anytime soon. My fluff will therefore be that it is the captains and Watch masters that undergo the primaris transformation.
Hope to have some painted Deathwatch up soon, I’ve painted one team so far.