350Mag MPI help me work thru this please

I have the above mentioned engine 0W666xxx, as clean and pretty a 13 yo engine as you'll ever see, and here's my problem. Fuel injectors 5,6,7,8 don't fire. Using a DDT, I cylinder drop all eight and those four don't drop. I load test the injectors, and the back four, two either side, don't test. DDT says "unable to perform test" or something to that effect. 1-4 click happily away when tested.
Studying manual #36, I see that the injectors fire in pairs, 1-6, 2-3 4-7, 5-8. There is 12v power to all eight, and the ecm grounds them to activate. 2-3 apparently works fine. 5-8 accounts for those two, if, for instance, the ecm has failed. But how do I account for 1-6 and 4-7, since 1 and 4 work fine?? It would require several different harness failures to account for it all. All eight injectors have all been cleaned, and while they flow a little better after cleaning, they all worked on the test bench before they were cleaned. I really expected four bad injectors, because this motor suffered from a large dose of water from the fuel tank. CF3 fuel pumps survived it. Motor actually runs deceptively well until you push the throttle, and it won't push past 3k rpm as you might expect.
Do any of your mercruiser gurus have any insight into what I might be looking at here? Sure doesn't seem like it should be an ecm. Seems strange also the pattern of failure, back 4 vs front 4. Thanks, Mark ZZ03

I would agree that what you are seeing is rather unique.

Going on the assumption that all injectors are functional, I'd be inclined to verify continuity in the harness, between the pairs, on the ecu side....and I would use a test lamp (vs a DVM) that will draw an amp or two. the 5-8 pair may be an outlier.

When last at the boat I didn't have the manuals or the wiring diagrams, so it's my plan to return and go thru the harness now that I understand it. Film @ 11 Thank you

Here is some insight into my confusion, although I now have to go back to the drawing board and find the solution...

From service manual 36:
2005 and newer GM small block V8 engineswith an ECM 555 processor (serial number 0W310000 and above on a sterndrive models; serial number 0W390000 and above on inboard models) will have four injector selections available on the Active Test screen in the CDS G3 toolbox.These newer engines have four injector drivers to fire the injectors in pairs.

I am using a ddt, and it shows it possible to test all eight injectors individually, but I'm now thinking that because of the way the ecm is configured, the first 4 are the only ones that respond, although this reasoning doesn't seem help with the 5-8 pair. 1 thru 4 in this case may refer to the 4 drivers, not specific injectors. I'm going to get to this motor with G3 and see what it shows me, maybe I'll get to the bottom of it.

In that manual you will also see the blurb that says PCMs can control individual injectors, ECMs control banks of injectors....

if you have four 'pairs' of injectors, there is no way I can conceive of to control them individually.....

I'd be inclined to make another assessment of what is and what isn't working....