'06 25hp EFI Merc 4 stroke Overheating

[sig_alph97]

So far my father and I have replaced the water pump impeller, removed the T-Stat. The TellTale appears to have low pressure and gets very hot (hot enough that I would not hold my hand under it). We get the red light and a steady tone when idling and moving. I am concerned there is a blockage somewhere. Used in freshwater only. The T-Stat will be replaced when part arrives. Any suggests to find the blockage? Should I replace the entire water pump (housing, etc.), des the water cooler plug? Is it safe to use an air compressor to try to clear lines?

Thanks