First you need to shut off the raw water supply at the hull fitting. Locate the pump, mine is mounted on the starboard side of the vessel even with the water line, yours could be on the opposite side. Remove the four 1/4″ bolts that fasten the pump to the generator. The only things you really need to take off of the pump is the cover with the o-ring and then get the old impeller out of the pump.You might have to slide to pump partially off the drive shaft and then back into position to get the impeller to stick out a little so you can grab it with your pliers. If you have any problems getting the old impeller out you will have to remove the whole pump to work on it.
If you have most of the impeller vanes on the old impeller there is no need to worry. If there are only a couple of impeller vanes on the old impeller it is a good idea to flush the system with a garden hose to make sure you have a good flow of water and most of the vanes are out of the system. Check the rotation of the pump by touching the start button on the generator control panel and observe which way the shaft has moved or if the impeller still has vanes on it install the new impeller with the vanes running in the same direction.
The next thing to do is to install the impeller in the pump. You should lube the impeller with Vaseline or liquid soap for ease of installation and start up. The easiest way of getting the impeller in the pump is to pull the pump from the generator a little bit, place the impeller on top of the pump and twist it in the proper direction to get it started (vane tips are trailing rotation) then align the impeller with the flat(s) on the drive shaft and slide it into the pump body. After the impeller is installed the o-ring needs to be placed in the groove in the pump housing with a little bit of lube on it and the cover replaced with the bolts and tightened.
When the cover is in place and tightened it is time to open the valve for the water intake, checking for no leaks. After all looks well there it is time to run the generator to make sure the whole system is functioning properly. There should be an ample supply of water coming from the generator exhaust port while the generator is running. This will become a normal yearly maintenance job on your vessel with normal generator usage.