Question
how thick does  a concrete lining of a pond have to be? also what mix would i have to use??

Answer
It depends on where you are located.
In New York, we  usually build them with a one foot thick wall.
It is also reinforced with a frame of epoxy coated 1/2 inch iron bar
In warmer states it could be much less.
Use a concrete mix with a p4 type cement

Good Luck!

Carlo