I have ponds, but not a lake. The biggest challenge with ponds is keeping them in water during a drought, but hopefully that won't apply to a lake in the same way, although I'd advise you to check the depth and the recent water level history. Ponds and lakes need management, with regular and often costly maintenance tasks, such as weed clearance, dredging etc. It would be worth discussing this with your insurers to ascertain what conditions they would place on it and how it would impact upon your premiums. Handing it over to an external body would be a smart move.