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I'm not able to call right now but would be glad to work with you here. Let me know.
Sorry for the delay. I'm here
OK. To be honest chances are the engine is frozen and may not be repairable. Lets try a couple things.
Remove all the spark plugs and using a penetrant spray liberally spray in all 8 cylinders. A great penetrant is a 50/50 mixture of ATF and acetone. Let the penetrant work overnight, then repeat.
After letting the engine soak for at least 24 hours get a large breaker bar and socket and put it on the bolts of the harmonic balancer. This is the pulley at the front bottom. Hopefully the engine will move a little. If so repeat the lubricant, let it sit for a few hours and then try the breaker bar again. If the engine turns over keep turning it while a helper sprays the lube in the cylinders. Do this until the engine turns freely.
If the engine is still stuck then you are facing a replacement engine.
How is the rest of the boat and accessories?
Try what I posted and post back and we can go from there. If you need me I'll be back in the morning EST.