Many marine engines with DI eg E-TEC and Optimax are requiring indexing, no big deal, it take just a bit of additional prep and time.
First make sure you have the correct tools for the particular engine you are working on, that may include a thin wall long 12pt socket, indexing washers if available and an accurate torque wrench. Marking the socket to the ground electrode accurately is easier with just a socket than a universal/extension setup.
Torque wrench accuracy should be within DIN norm which is 4%, eg a 23 nm value will be between approx 22-24nm, this is normal. I just bought a new Norbar torque wrench, PM me if you need the link to it, this one is 3/8 12-50nm and is about 0.5% - 1% (spec is 3% or less) for $120 and its made in England, perfect for work like this and a very nice tool to own.
I will post pics and info in another thread as soon as I have more time.
Do not use anything but an OE MB plug, do not use NGK, its too short for one thing, a picture is worth a whole paragraph.
The way I do this job is to first accurately mark the socket to the ground electrode all the way up to the top of the socket, do not use any anti seize on these plugs, they will not be in the engine long enough to carbon seize anyway.
Using the 23 nm as an example set the torque wrench to 22 nm and tighten slowly and steadily once the plug contacts and see where the electrode is in relation to the injector if it is slightly before then tighten to 23 or 24 nm to bring it to center or if it within 45 degrees after center leave it at 22 nm.
If 24nm will not bring it within 45 degrees loosen the plug and tighten it again, sometimes this will crush the washer just enough to bring it within spec. If not use another plug or the washers from the old plug or an indexing washer if available (these are sold by thread diameter not vehicle specific).
Do not beat yourself up trying to get it exactly at zero, it may not happen but it does need to be close. This is not a complex or difficult and easily doable by DIY.
PM if I can be of any help or you need clarification or links.