What type of nose stud is recommened to have in when you're swimming?

As far as material, I suggest something non-porous like steel or titanium. Glass isn't porous but its pretty fragile. If you have a ring that closes like a segemtn, seamless or CBR, that would be less likely to fall lout compared to a nosebone or even a nose screw. Just make sure that you clean the piercing thoroughly after swimming :)