um, who are you buying through and for what car where it DOESN'T include the water pump housing???
every pump i've ever sold at work has the housing & gasket (at the very least, of course) and sometimes with a pulley (usually for vehicles that have pressed on pulleys)..... i would never buy something so cheap that you basically have to rebuild a water pump.... new ones are cheap enough as it is.....
I think he's got a 7AFE AE101. Some parts places are weird, I've called for parts before and they have asked me the same thing for other parts, once they get the parts they find out.. "Oh, it came with the gasket" or whatever. I guess some parts probably come separate for other cars so they assume they're all the same.
On the 89-92 Corollas, the body of the water pump comes in two halves. One half has all the moving parts. The typical new water pump consists of only this half. You also get the gasket that is used to join the two halves. If you don't trust your ability to get the two halves sealed or in the rare case that the other half is damaged, I suppose it would make sense to buy a "whole" pump.