454 Mag overheated now not running right
 

Here's the scenario, We were out for a little cruise yesterday and before I could notice my starboard engine overheated. It was pegged at 250 degrees, so I'm not sure exactly how hot it got.
Come to find out today that it was the impeller that went, easy enough. But now that motor doesn't sound right, it's kinda surging and loping. I'm not sure what;s going on.
Suggestions?
Did I happen to mention that I am getting married in 17 days and wanted to use the boat in the wedding:readinghelp:

Do a compression and leak down test...

first i would do a compression test on it. see if you have compression even across all cylinders. you might have warped a head or popped a head gasket. could have also melted a flapper in the exhaust and you are getting a hindrance or flow. you can check this and see if you are getting hotter on one side of your exhaust than the other.

Yes I am getting some water flow through the pipe but it doesn't seem to be as much as the other side. Also there are no flappers in the pipes.

I agree