Here’s a reference thread, along with some diagnostics that may be useful to you, in the event you have not already seen them.
427 Cooling system diagrams
http://www.network54.com/Forum/424840/message/1123102009
I suppose someone in their younger gymnastic years could do this impeller change-out with the pump still on the motor, but it would take standing on your head for a while in a very uncomfortable position on my 38 Express. Maybe other boats have better access to this equipment. If you’re going to do the job right and look inside, inspect things, you’ll need to pull the pump out and get it on a bench.
Here is a complete pictorial of the nsides of a 427 Sherwood sea water pump
http://www.network54.com/Forum/424840/message/1156595176
A look inside a Sherwood sea water pump. This photo shows the replacable cam, which is what actually creates the suction on a pump of this design. These things can eventually wear down to the point where they produce less suction, even if the impeller is new. I think they're about $35, and still available, which is pretty amazing.
Paul
