MAN D2842LE410 - Sea Water Pressure Regulator

You don't need any repair kit.
It's sufficient to strip the pipe and replace its special connecting screw with a plain vanilla screw.
The only trick is that this screw must be long exactly as much as necessary to keep the diaphragm stuck open.
Can't remember this exact length by heart, but any half decent MAN mechanic should be able to advise.
Don't forget to replace also the connecting screw at the other end of the pipe, connected to the pressurized air intake.
But this is even easier, because you only need a very short screw, just to plug the hole.
That's all you need.

I meant to say previously, but I forgot.
Beware, the rationale behind that valve is NOT reducing startup smoke, as mentioned in the old post preceding yours.
The smoke reduction upon cold start that the valve can grant is anywhere from almost and completely unnoticeable.
What the valve is really designed for is avoid cooling charge air when the turbo is not spooling, i.e. at very low RPM.
In fact, in those conditions, and with a cold sea water temperature, the air feeded to the engines could get excessively cold.
Bottom line, if you are using your boat a lot at displacement speed and in cold waters, it makes sense to have a fully functioning valve.
Other than that, it's useless and you can lock it open as I previously explained.

Whoa, the electronic control units submerged?!? Good luck with that!
I can send you the PDF of the MMDS repair manual if you wish - just pm me your email.
But pretty sure you will not find any indications of what to do to recover an electronic unit which is supposed to never be exposed to condensation, let alone a full immersion!

And ref. starting the engines without the elctronics, forget it.
All you could possibly do is power the starter motor and crank them, but the whole injection system is 100% electronically controlled, and no fuel will ever be squeezed in the cylinders unless commanded by the engine ECU.

PS: if by chance you're only interested in making a 720 degrees engine rotation for checking that there isn't any stuck valve, that is possible also manually, with a long ratchet wrench, a 32mm hex socket, and a bit of patience.

Failed positive displacement test???
Don't think anybody mentioned that before..

What do you mean "after" 5 hours?
I assume they are smokey all the time, because if they aren't for 5 hours and they become smokey only after such long usage at low rpm, that's something unheard of.

Regardless, I very much doubt that the smoke is due to that valve stuck open.
You should understand what that valve does, which as I briefly explained in post #11 is "only" to avoid an excessive air cooling, and keep an optimal charge air temp also at low rpm.
But with MAN engines, once the whole block is warmed up, their typical smoke upon cold start should disappear almost completely, regardless of whether that valve is fully functional or not.
The real risk is in fact the opposite: if the valve is stuck close, the intercooler does close to nohing, and at high rpm the EGT goes ballistic, overheating the engine badly.

If you are constantly smokey at low rpm, my guess is that you have injectors problems, which either need calibration or nozzles replacement (or both).
You don't mention the model: are they common rail or not? Hours clocked?

Your engines belong to the so-called EDC family, which were produced for just a few years as a development from the previous fully mechanical engines, before eventually adopting common rail for the whole MAN range.
AFAIK, their injectors and HP pump are similar to the mechanical ones, but the pump is driven by an electronic governor.
Now, I'm not sure of what you mean exactly by "rebuilt", but if the injectors and pump were pulled and serviced, I suppose they were all calibrated and fitted with new nozzles, so they shouldn't be the culprit as I previously envisaged.

Now, I'm still a bit skeptical that the valve alone, even if stuck completely open, can make a big smoke difference.
Yes, the principle is that the valve should open when the turbos are spooling and the charge air needs more cooling, but at the same time it shouldn't be cooled too much, negatively affecting combustion.
On the other hand, it's not like you're boating in the Bering Sea...!

Maybe it's just a matter of not exaggerating with low rpm cruising.
I mean, 700rpm is barely above idle, and 5 hours is a helluva lot of time!
Besides, I'm not positive on EDC engines, but I believe they also run with fuel injected only in one of the two cylinder banks, up to about 1000rpm.
So, I suspect it isn't just a matter of how cool the charge air is, but it's the whole block that never reaches its normal operating temperature.
What coolant temp do you see on the instruments, during those 5 hours?

Anyway, one test you could try is running at the normal cruise speed those engines are designed for (anywhere between 1800 and 2000 rpm) for some time - say 20 mins minimum, better 30 or more.
Coolant temperature should stabilize around 85/87 deg Celsius, and smoke should be pretty close to unnoticeable.
In these conditions the engine are fully warmed up and running at their optimal operating temperature.
Done that, drop the speed to 700rpm and check if the engines immediately become as smokey as when you're running for a lot of time at the same speed.
I'd be very surprised if they would, while if (as I would expect) they don't, it's good news/bad news time.
The good news is that there's nothing wrong with your engines, and you can not bother replacing the valve, because the fact that it's stuck open is practically irrelevant.
The bad news is that you must change your cruising habits, and forget running for 5 hours in a row at idle, because that doesn't allow the whole engine to stay warmed up as it should.