Pond repair

I have an old cement pond that has started leaking after 60 years. I do not
want to replace it with a membrane, both because I dislike the appearance and
because I do not want to disturb either the well established margins or the
large planted island in the middle of it (it is a large pond maybe 40' long).

I have tried patching, but this either didn't take or new cracks appeared.

I am thinking along the lines of a layer of reinforcing mesh and a skim coat of
cement to seal it up again, and while I am at it, I'd like to drop one section
down a couple of feet to allow a place for the fish to hide and the lilies to
grow - the pond is fairly shallow.

This chore seesm to fall between the areas of competence of the liner pondies
and a swimming pool company - no one seems to be geared up for the repair of
existing ponds, though many will offer to install a new one with membrane,
which is not what I want.

I am prepared to have one more try at patching, if anyone has any suggestions
for a flexible patching compound that will bond to old concrete. If that
doesn't work, I guess I may have to talk to the swimming pool people :-(

Any brilliant iedeas out there?