I may be late to the party but had to replace mine last month and should share my learnings...
Bought only the pulley/impeller housing half of the pump and new impeller from MarineEngineParts.com for $165.69. It is a Sierra part. AutoZone has a tool loan program and I picked-up their GM power steering puller to remove the pressed on water pump pulley. Long story short rather than paying attention and repositioning the puller, I threaded the puller into the shaft hole of the pulley ruining it. New pulley was another $64.94. Lucky I didn't have to buy the AutoZone tool...
When my pump went out it was leaking water around the shaft. Water would drip on the belt causing squeal. I only replaced the impeller half of the pump because I am too cheap. And I didn't dissassemble the old pump until the new housing half arrived. When I did I realized that the rear housing housing half had damage from the shaft floating around due to the bearing/seal failure. Not a problem now but if I had seen this I would have purchased a complete pump. Also has me wondering if the cause of the whole problem is possibly my over tightening the belt following my annual impeller change.