Just had my water pump totally rebuilt professionally, including pump body, because old one failed after engine rebuild and showed signs of over-heating when taken apart. I have never run OB without water, and I doubt that marine shop did either. I check tell-tale every 5 minutes during use. On the few occasions over 11 years when it has ceased, problem has always been blockage at tell-tale nozzle. However, I have only gone 3 hours at moderate speed on rebuilt engine and pump, and yet pump sounds like it is failing. Tell-tale jet was strong after rebuild and throughout the 3 hour run. Only thing I can think of is a partial water intake blockage. Not enough to stop water circulating well around engine block, but sufficient to reduce water flow through pump so that it overheats.