I've been down the road of a leaking pond and still trying to solve the puzzle.

Given the conditions you stated above, I would go the route of a liner and use an underlayment as an extra barrier because of the rocky soil. If you have boulders, clay won't do much good unless removed or buried deep. I wish I would have gone this route, but was talked out of it by my 2nd contractor. I have used a lot of polymer and would not recommend it.

Rolls of liner weigh up to 2500lbs so a mini excavator or bigger skid steer will be needed to unload. You will need to dig a ditch on top of the bench 6-10 feet from the high water line and "tuck" the liner under 18"-24" of soil to hold it in place. Slopes should be no steeper than 3:1 or less so it doesn't stretch and tear with the weight of the water. Welding the seems is key to the install, so be sure to take your time.