Re: 1998 Gamefisher by Mercury - Water pump question
It's a Force outboard, Force was bought out by Mercury (explains tag in the cowl),,, and then rebadged by Sears

You can find part numbers and schematics at Sears (don't buy the parts there) or Crowley Marine, Iboats should have the impeller in the store.
If in doubt replace the impeller as it needs to be changed every 2-3 years anyway. While you are at it , you might look at changing the fuel pump ($4 kit, diaphragm and gaskets) as ethanol is hard on the old rubber parts.
There is no "telltale" on these outboards, but you should have some water misting out of the exhaust relief holes in back of the leg.