You might start with this post
http://www.tugnuts.com/viewtopic.php?f=2&t=3732&hilit=raw+water+pump and then search more for the discussion on what people say about the frequency of replacing the raw water impeller.
Good luck. On the R-25 (Classic) 4BY2-150 the replacement task in non trivial IMO.... especially if it has not been done at least once.
[EDIT]
The frequency of inspecting/changing the raw water impeller is somewhat of a personal choice.
The impeller is a critical component for the well being and health of the diesel engine. If the impeller fails the engine will lose its cooling feature and overheat very quickly. If this were to happen while on the water then the task of replacing the impeller is far more difficult than when docked or on the hard.
For the R-25 (Classic) the effort to replace the impeller is a non trivial task and even for an experienced technician can take a few hours. If yo've never replaced the impeller and you DIY the allow many more hours. For the larger boats and maybe for the R-25SC the access to the impeller is easier but still will take some time.
If you worry about the impeller's age or effective state for doing its job and want to ensure it does not fail at a critical time then I would replace the impeller at least every 2 years or every 100 engine hrs. Again, this is a personal decision for you to make. Even starting with a brand new impeller there's no telling how long it may last. Boating in water that has silt or fine sand floating about will likely wear the impeller vanes quickly compared to fresh clear water.
Pulling the pulley and removing the impeller housing front plate to inspect the impeller vanes is not sufficient for determining if the impeller vanes are in good shape. The visible front surface of the impeller may look fine but the backside could be a different story. If you go this far its best to simply go ahead and replace the impeller as half the task has already been done.
If the engine has not been run for many months or even a year or so I would say it would be prudent to replace the impeller as having an impeller being dry for extended time is not good as the vanes can become hardened and more easily cracked/split when being used after being dry for so long.
Always ensure that water is being expelled from the wet exhaust. There should be a fair amount of water being pushed out. If not, then it's time to inspect the raw water cooling system. Is the hull's raw water pickup grate blocked ? Is there water freely circulating in the raw water strainer ? Is there seaweed/crud piled up in the raw water strainer ? Is there a blockage in the raw water hose leading to the impeller pump housing ? Is the raw water pump pulley belt slipping or has it snapped ? Finally, inspect the raw water impeller for proper functioning.
