It will eventually pit and rust most likely.
I work in an OR and its a big NO NO to put the instruments in saline (saltwater) because of the damage it causes them. They are only placed in WATER.
now there are different "surgical steel" available. As far as the steel in some implants I would say they may be stronger to an extent due to the mixture of metals, and possibly a finish on it.
Is there something in particular you are looking to use it for?
Jess