Cast stainless with low magnetic permeability

316 (A352 CF3M) can be cast with high nickel and nitrogen to acheive <1.05 permeability. However, it is not the best choice for seawater. A352 CK-3MCuN (254SMO) would definately be a better choice for seawater, and although I haven't seen its permeability measured, I know it shows up with 0 FN on a fischer ferritscope.

We have had some experience in the past with Ni-Resist (I believe D2C). From what I was told the castings had to be completely machined or ground after heat treatment (900°C+air cooling). If this was not done, the low permeability was not attained.
This contradicts with publications I found stating that machining will induce cold work and thus the formation of martensite. This would raise permeability. It also contradicts with a publication stating that a solution annealing lowers permeability.

Unfortunately 254SMO is not an option due to price issues.

If we only consider CF-3M/CF-8M or Ni-Resist, what would be best?

I don't have any experience with cast iron at all, but if you post a question about corrosion resistance in the corrosion engineering forum, they might be able to give you an idea of which would last longer in seawater.

We have used castings in CF 8M successfully in sea water applications. These were placed in water for periods extending over 1 year and no trace of corrosion was noticed. CF8 M offers a cost effective solution .