I've just made and filled a pond. This one is going to be a fairly
formal pond, and now has slabs cemented in around the edge.

The slabs are above the water level obviously, but I want to conceal
all the liner. Problem is that I don't want the water in contact
directly with the cement because it isn't sealed. So the question is,
what should I use to make a waterproof seal all around the pond, above
the liner and just below the slabs so that no liner is showing?

If anyone could recommend a product or technique I would be very
grateful.

Will