It is on the lower right of the exhaust cover on the back of the powerhead. It's the thing that looks like a thermostat housing in this picture.
http://www.monkeybutler.com/boat/yam...n/IMG_4548.jpg
There is a rubber grommet in there that you should replace as a maintenance item along the same schedule as your t-stats. Also, Yamaha changed the design later on so it was less likely to get stuck open by debris like sea grass. If you go to
http://www.yamaha-motor.com/outboard/parts/home.aspx
and browse to your motor look at the tab for "CYLINDER CRANKCASE" and parts #48 to 51 will get you the new design.
Also, if the pump was just done then when you pull the lower unit check to make sure that the rubber grommet that the water tube plugs into up in the mid section is in place or you could lose water pressure there too.
Steve P.