You're going to see shafts in grades of 17, 19, and 22. Go with the A22; it's a bit more expensive but has superior corrosion resistance and is strong enough in the diameter of what you need which I'm assuming is 3/4".
Thanks for the replies. I am not familiar with Aquamet22 but the SS316 is what I'll use I guess. It's for a 7/8 shaft, 49 inches long on a 15 hp yanmar