You can just pull the cover and replace the impeller. As you have noticed the pump shaft goes into the engine
cavity which means that if the seal behind the impeller goes, you can pump seawater into the sump. If my memory is still accurate (Its 20 years since I had to work on a Green Death) , you can drill a couple of drain holes in the pump casting to drain any water
before it gets into the sump, but ask someone knowlegeable before doing the drilling. Regards, Richard.