90 HP Yamaha overheating

[cm45]

I have a 1989 Yamaha 90HP with very low hours that was not used for a year, to make a long story short it overheated (horn) when first started so I ran salt away through it which stopped the horn. Decided to replace impeller, which was missing some rubber so then ran the boat and the engine over heats at WOT. Pulled water jacket and the head, cleaned all salt and replaced the thermostat. Ran the boat and still over heats at WOT. Pulled exhaust plate covers and cleaned salt also found some rubber from impeller cleaned all, ran the boat, still overheats. I give up, any suggestions??

[boats.net]

Probably need to replace all of the water pump, housing and water pump kit.