Been there... luckily with a smaller engine. Sucks with any size engine, but certainly more involved as things get bigger.
Last summer, my son, barely 15 years old, and I pulled an old, 13' skiff out of the woods where it had been sitting on our property in Maine for a couple decades. It was time to have a dad & son project. I had just gotten an old 2.2hp Merc for $50 and it was like new - ran great. So, we carry the boat down to the water, put the engine on... could tell right away that the transom was soft, but it seemed to be holding well enough for the hamster-powered engine. So, away we go for a little exploring on our "new" boat. Ran around for about an hour - he tried his hand at his first time with a tiller steer - that was "fun"! All went well. Back to the dock we go.
I tell Cody - hey, why don't you take the boat out by yourself! He is hesitant, but finally decides, with some prodding, it would be fun. I sat on the dock to watch him, eating a snack... actually it was Capt'n Crunch in a ziploc baggie... and truthfully, it was his snack. Anyways, he pulls away from the dock and literally, within 20' of the dock, he makes a hard turn and the engine goes ker-plunk! He was a captain of a power boat for all of 8 seconds.
The boat, and Cody were facing away from me. He just sat there - refusing to turn around towards me. I think he thought I was going to be mad at him. I just kept eating the Capt'n Crunch - it had been a long time since I ate some of that and it was tasting awfully good! He continued to just sit there, not facing me, and I continued to eat. How could I be mad? He didn't do a thing wrong. It was actually kind of funny! Finally he turned around, after what seemed like an eternity, although was likely only a minute. I just asked "Hey, bud, what happened there?" He didn't know what to say. I told him to oar in and we tied the boat up. I gave him a hug to let him know everything was OK. We started to walk back up towards the cabin, but I paused and said "Guess who's going swimming tomorrow at low tide?" He smiled, grinned and simply said "Me". THAT was a great freakin' day!
A few days later, we rebuilt the little transom together. And after thoroughly cleaning the engine again, it was back to running in tip top shape. This time, though, I added a small, stout cleat to the inside of the transom and tied off the engine to it...