400 series is not suitable for subsea or ocean, but it more used for surgical steel because of the hardening capability.

400 series does not contain nickel, and generally for stainless characteristics you are looking for combinations of chromium and nickel but with 400 series it is chromium that gives the corrosion resistance. Also, 300 series are not magnetic (a trade of austenitic steel).

What is it you are designing and the constraints? The other problem with 300 series SS is the fact that they require oxygen to work, that is to say they need oxygen for the surface oxide to protect the metal, without it the steel will corrode, so 300 series is susceptible to crevice corrosion.

For stainless steel informatin, you can try this site: http://www.ssina.com/index2.html

If you want, you can contact me at jason (at) jasedesign (dot) com