Starting bad noise/5.7 Mercruiser

[oceanpearl]

Yesterday was the worst. For the last couple of times we used the boat, there was a hesitation and a weird bad noise in the middle of the starting cycle. I would shut it down and restart with no issues.
Yesterday the problem persisted after the 2nd and 3rd and 4th attempts. At one point it tried to start but stalled out.
When it did finally start, the serpentine belt did not move but the flywheel was turning and the belt was spitting dust. Shut it down immediately.
The mechanic removed the belt and tested all the pulley wheels but none were seized.
After that he had normal starts for 25 times.
Anyone have any ideas what the problem could be??
Much appreciated.

 

[scoflaw]

Improperly adjusted belt or the water pump. The impeller makes it hard to turn, so you can't spin it freely assembled

 

[bobeast]

Take a look at this bulletin. I was shredding belts like crazy until I added the spacer mentioned.