Welcome to the forum!
Try calling these guys. I don't know how close you are to them.
https://lakework.com/Update: These guys are very reasonable and appear to offer the services I need regarding the dam and overflow inspection. I'm waiting for go ahead from current owner.
What services would you purchase at this point in the process? I would really like to know the depth/average depth. What else do I really need/want to know before purchase?