Bought a boat with a blown engine (not knowingly)...fluids in the wrong place. Why?

Are you sure he put fluid in the engine and not an oil addative like say Marvel Mystery Oil ? A marine engine can be very costly. The best thing to do is sell the boat now before you get involved in it. Part it out and you should get your money back in parrs. If you are not mechanically inclined in will be very costly for you to own a boat. They always break down and you will always have problems. Even new ones give you problems. If you keep the boat get a rebuilt engine it will cost less. Good luck.