Unfortunately, the 75hps are more susceptible to powerhead damage than the regular 70 models, so I’m guessing there is internal bearing/journal damage. For now, I would recommend reinstalled the head/gasket (if it looks OK), then removing the gearcase. Once the gearcase is off, you can start the engine to expel the water and clean off the bearings without worrying about additional water entry or impeller damage. Once the powerhead has been somewhat preserved, you can take your time pulling it apart.
And yes, it is best to pull the powerhead off when trying to remove that exhaust cover mess, you will probably encounter a few seized bolts.
Sure hope I am wrong about the bearing/journal damage, but I fear not…
