Apparently my water pump failed causing my engine to overheat and melted my piston rings (according to a local mechanic). Is this fixable? If not what is the best way to go about finding a good used or rebuilt engine?

I am currently stuck at work on the river and won't be able to get off the boat to deal with this directly for another 3 weeks. I'm in a huge bind now. I just paid this damn thing off and registered it two days before this happened!

Any advice is greatly appreciated.

Edit to add: the engine will still turn over and I believe it may have actually been caused by a cheap camshaft synchro I installed because of the high pitched chirping sounds it began to make right before this overheating.

 

Get a 2nd opinion
Overheating its cause by low coolant, so not related to Cam sensor
You can melt pistons from pre-ignition, pinging and knocking, sounds like a diesel engine, but needs to happen for awhile and you WILL HEAR IT

You an crack/warp heads, burn exhaust valves if an engine overheats, never ever heard of melted rings

But if the compression is low then more than likely the heads will need to come off and be replaced, you can then look at the piston and rings

 

It did begin to definitely knock firefly before it completely died. I put coolant in it but it only made it a mile or so before it began knocking and overheating again.
I think there was a long standing compression issue on at least one cylinder due to consistent misfires (cyl 2 fires at twice the rate of the others). I'll have it towed to another shop for a second opinion as you suggested.

 

If it was pinging/knocking and overheating, and you continued to drive it then you could have melted a piston or two
Did they look inside a cylinder, i.e. bore scope?