Question
I am building a water fall out of flag stone to my pond and want to know if there is an additive to put in the mortar to make it more water proof?  I am planing on using quick crete mortar.  Any advice in this area will be helpful.
Thank You

Answer
No mortar is water proof or even water resistant by itself and no additive will be sufficient. You must have a water proof liner or membrane under your mortar that slopes into your pond reservoir. This way any water moving through the mortar will hit the liner and drain back into your pond. Water can move through mortar very fast. You could loose all your water in a day or less in some cases, specially where mortar meets rocks.
You can seal your mortar and make it water resistant, but some water will always get through. It is always best to have a liner beneath. As a designer and contractor I would NEVER build one without a liner.