For 40-bucks,
www.faysboatyard.com sells a 200-lb block of nicely finished concrete with a 3/4" id pvc tube set inside for an el cheapo 3/8 or 1/2" yellow floating polyethylene mooring line to pass through. Boaters pick em up from Fay's by boat and roll the mooring block overboard onto spot-x.
X marks the spot!
No nothing to corrode, four 90-degree side long corners to catch & hold; and the concrete/pvc is warranteed good-to-go in Lake Winnipesaukee till the year 2525 by Merrill Fay, himself!