This has nothing to do with your pump shoe from what your explaining. What's happening is due to riding salt water unfortunately, although it could happen in freshwater. A wear ring is basically an aluminum housing with a stainless steel liner, and corriosion ends up occuring between the two causing the liner to bulge or become uneven. You're gonna have to buy a whole new wear ring, and for saltwater one with a replaceable liner could be useful.
@JetManiac sells exactly what you need, unless you go the route of picking up another used OEM wear ring off of the classifieds here. Your prop may need reconditioning as well, especially if it's been hitting on that bulge for some time..