Not too worried about cracks, could remortar or there is a concrete caulk sealant....I do have big pond (preform we had there, it was ok for 2 years, then the drain started leaking the year I let it freeze solid, so I removed it (plus the geese would "guard' it from the ducks, so am moving it nearer the garden and house. But it is too deep, I believe... I have read some breeders say a shallower pond cleans better, because they keep it stirred ... Partly because in the day pens, the tubs keep cracking (concrete mixing tubs)...
I like to do things the hard way ;0), Most of my work on my 11 acres is handled by me with a wheelbarrow, craftsmen mower with wagon attached, a shovel, a hoe, saw, hammer drill and pliers ;0)
so 3.5 inches would be thick enough, I 'm thinking freeform, maybe press some of the pond rock in it....Something I can bail water out with a broom and let the hose overfill it.....