Help overheating

I have an 83 correct craft ski nautique. On the hose at home eng runs fine... Normal temp, (I hypothesized this is from my house water presser pushing water through-ought the system)... put it on the lake and temp shoots up over 230. I have a t.stat that is 160 for PCM. I have tested the water pump and replaced the impeller, tested each connection for possible air leeks with neg/ results. Systematically using a 5 gallon bucket and timed the time it takes to drain the bucket approximately 20 sec @ 2500 RPM. The only thing I can think of is that my skimmer top is linking air into the system if there is any one with a PCM Ford setup like my 83, can you look and see if the skimmer has an O-ring for the top and is so any ideas if this is my problem since I have excluded the water pump as being my problem.. Thanks Rob

put running water near the rwp,at the shaft, if its pulling air this trick will clear it momentarily, you may have to do this for a few minutes for the temperature to regulate, the shaft seals are commonly overlooked and if pulling air will turn the water into airiated water which does not transfer heat