Yes a load of water can kill your injectors in the 60 mi drive you mentioned of the water separator didn't catch it.
Yes $7k for new injectors is not out of line since virgin injectors go for $7-800 each parts only.
No it likely was not bad injectors because injectors damaged in this manner don't fix themselves, although they could have been questionable and the computer wasn't triggering a code and going into limp mode until a certain set of conditions were met.
IMO this is a dealer issue if they hosed you on injectors, not a bad GM truck.
Still sucks I realize but if it really was bad injectors then you didn't get takn for a ride.
Can you be more specific about the problem with them? Bad balance rates, leaking/ fuel in oil, etc?
What everyone needs to realize is that high pressure fuel systems approaching 30ksi like modern common rail diesels need proper care and IMO a lot of the OEM specs and filtration is not sufficient to maximize fuel system life.
Just put new sticks in my ram earlier this year due to bad balance rates on 2 of them, causing them to not build enough pressure on cold starts resulting in hard starting. I didn't believe all the hype about additional filtration additives, etc until I lost 2 at 115k mi.
Difference was, new remans (all new parts, used bodies) cost $2300 and I was able to replace them with hand tools in my garage in less than a day. Immediately added 2 micron filter and use lubricity additives now and will be adding a pressure gauge and taking care of any lift pump issues that may arise and starve the IP.