There are two pumps on each engine, one in front of the blower and one usually on the back of the engine. The blower pump is coolant, the back one is raw water.
Either could be changeable by someone with some mechanical ability and common tools. I figured it out when I was 13. The front pump should have a coolant drain. The rear pump usually is above the water line and cracking the outlet will let the water drain out the seacock (if it's open). There should be a zinc in the raw water pump and removing that will also let the water out. Good time to replace all engine zincs. I get mine here:
http://www.zincwarehouse.com/engine-pencil-zincs.html Much cheaper than marine stores - about $3.
Depending on how old the engines are, there are two different styles raw water pumps. The old style have mostly been replaced. They commonly leaked and used brass/bronze gears and two shafts. The modern style uses an impeller and has an enclosed single shaft. All the pumps can be rebuilt and there are kits available.
Most likely, the shaft seal is failing. They fail with age or use. When I buy a boat, one of the first things I do is replace all the shaft shaft seals.
If I were you, I would buy a service/overhaul manual. They're often on Ebay. Most, if not all of the maintenance can be done by a careful owner.
If you're near a big city, a bearing supply should have the seals. Also online.
Ebay often has new/used pumps and is a good place for spares. I have 2 671s and carry spare pumps. If you get a manual, you will see, with some additional tools (drivers), you can rebuild the pumps without power equipment.
If you have turbo equipped engines (671TI, etc) you need high volume coolant pumps. Use a new o-ring if replacing the coolant pump. Buy new gaskets in any event.
