Impeller woes.

[Hammerhead]

The other day, I wanted to but.,couldn't go out on the water. I figured I'd burn the gas at least out of the carb while it sat on the trailer. I hooked up the ears,..started the boat. It ran at idle fine. Then I noticed a whirling noise. Then smelled,..Rubber???
Looked over the aft..Holy moly,..I DIDN't turn the water on. I hopped overboard to do that hoping It'd cool the impeller down plus the motor was now getting hot !!! AAARRGGHHHH.
Yesterday I took it out for a test and sure nuff it was running hot by about 20 degrees or so,..@190. There's water blowing outta the exhaust but, I think I gotta change it. Any thoughts?

Like I don't hava nuff to do,...this goes to the front burner.
Never changed one before. Any tricks if I do? It's a '77 merc-cruiser i/o TIA