My outboard motor was submerged in the lake, is it ruined?

Probably not. Depends somewhat on the cirstances. Is it a two stroke (oil added to gas) or four stroke (oil in crankcase) engine? Was it running (hot) when it went to the bottom? How long was it submerged? Two stroke engines are simpler to restore to working order than four stroke engines. If you check around on the internet you'll be able to find advice on how to restore a motor after it's been submerged. If you're not into doing it yourself you can find an outboard dealer/repair shop that will do it. Check with several places so you get the best deal (=price and quality of workmanship).