The bottom line is that if there's been damage, there's been damage. If it is OK. it is OK. Change the oil, run it for a day or two and change it again. You'll want to find out why it got in there, but I've changed more than my share of lower unit milk shakes and most of them have lived.
The oil actually still protects the steel against heavy rusting. The issue is if the lubricity of the oil is diminished badly enough that there is bearing damage.