I have never been involved in this, or even seen one, but I would bet that in order to pull it off, and pull it off well, a concrete base is best, taking care to mind your frost line. And as far as mortaring between the rocks, hydrolic cement is non shrinking and works well with water, I have used it to stop water leaks underwater and while dealing with pressure. (stopping leaking basements and manholes) Good luck.
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