How can I repair damaged shower enamel?
You could put enamel right back down again.  One common brand of alkyd enamel paint is Rustoleum, commonly referred to as an "oil paint".   You'd want to keep it dry days before and after the repair.  

For a more durble repair, use a 2-part epoxy paint.   Those paint systems have a fairly high entry cost, so I wouldn't do it for that job alone.  But if you already have access to it relating to something else you do, it can't be beat.