If you could only get one fixed ND filter...

[Skip70]

Now that I've learned about the deadly Dark Cross Effect of a variable filter at wider angles, I'm thinking of getting a fixed ND filter to take down most of the light and perhaps just use my variable on top to fine-tune a bit.

With that in mind, what density would likely be best if I'm looking to get moire of the soft water look on gentle ocean swells?  I was thinking eight-stop, but what does the expert community say?
(I have a 6D MII, so my minimum ISO is 100. I generally don't want to close down aperture more than f11 to avoid diffraction.)   

[rs-eos]

I rarely worked on such photos, but I remember mostly using a 10-stop ND at the time.  Though on occasion, also used a 6-stop ND.

Since you don't want to stop down beyond f/11, I think something closer to a 10-stop would be better.  Though it really is going to depend upon what final effect you're after.

I'll also assume you have a good tripod for this work and potentially a shutter release cable.

Other notes: with something at or near 10-stop, extremely doubtful you'd be able to acquire focus with it on.  So you'd need to acquire focus first, set to manual, then attach the filter.  I have usually been able to acquire focus with a 6-stop filter on.

Finally, beyond long exposure, do you foresee using the ND filter for other things? e.g. I'll use either a 3-stop or 6-stop filter when wanting to stay away from using flash High Speed Sync.  For that work, I never employed a 10-stop.