Ok so cause the cache can be retrived with out getting wet it lowers the difficulty.
I want to make it difficult to find and not easy to be spoted due to muggles on the lake, so I would probably use a small plastic tube made to look like a log as the buoy, motored boats are not allowed so I shouldn't need to worry about the cable being cut by a propeller.
Probably not swimable the nearest point of land is 200m away and it's just a small island, the main land is averaging about 500m.