On my boat, B36(2002), the shower drain is fitted with a filter located under the wash basin between the shower drain and the pump. No one has suggested this, but have you checked that the filter is clean?
The filter bowl unscrews from its base and contains a tubular wire mesh grid within. The mesh grid prevents all kinds of crud from getting into the pump, and if left untouched the filter would eventually choke off the flow of water until it was next cleaned out.