I've had occasion to repair both in the past.
To repair a crack that was threatening to destroy an above ground cisterna, we used a big angle grinder to dig deep into the  crack and expose it to at least 5mm wide and 20mm deep. Then cut slots across the  crack for about 75mm each side and 20mm deep again. Next is to set rebar into the new slots. For extra grip, you can drill holes at the outer ends of the slots and bend the ends of the rebar to sit into the holes to make a secure staple. The diameter of rebar will depend on the severity of the crack but use at least 8mm diameter. Set the rebar straps  in place using the epoxy compound in a cartridge gun. Use plenty of epoxy to fully enclose the rebar. Use the epoxy to fill the entire crack back to about 5mm from the surface. Fill the remainder with sand & cement. If accessible, do both sides of the wall. Then follow on as below...

For a leaky or seeping cisterna, use Osmocem. This is a specially loaded cement product that is completely waterproof. Comes in 25kg bags at about €50 a bag! I bought from Gueches in l'Ametlla. Fortunately it goes a long way. clean the surfaces thoroughly, a pressure washer and stiff scrubbing brush will do.
Mix Osmocem with water using a paddle in a drill. Wet the surfaces well with water but mop up the excess. Paint it on thick, brushing in 2 directions and working well into the surface. I did a 6 x 4 x 3 cisterna with a bag and  a half. A cisterna that was losing a couple of cubes a week is now not losing a drop. That was over a year ago.
Obviously for a pool it means stripping off the tiles where needed. Take off more than you need as water has probably got behind more tiles and they will just come loose anyway.

Any more ideas or suggestions, please contribute.