Look on the cover plate of the pump, it often has pump mfr and model number. Or cast into the pump body. Impeller number can be looked up from that. Or take out impeller and often it has the impeller number molded in. It is not uncommon for marine engines to use pumps from different suppliers, thus the confusion.
Popular pump brands are Jabsco, Sherwood, Johnson, Oberdorfer, and some others. Good chance yours is one of the above.