I suggest that you try posting in the pond section of this forum, You'll get better responses. epandable black foam works well for thin cracks, if your doing a major surface you can put down pond liner and then use whatever you wan't, Just avoid letting runoff into the pond with fish, its got a really high pH.

The pond folks could probably help you out.

With my pond, which does have a waterfall, I don't use any kind of sealant - essentially because I like to make changes in the pond, and don't like the idea of keeping a static form.

Making a falls without leakage takes a bit of time. You need to have the right stones and also need to be cognizant of slopes and angles. It can be done depending on the design and stones/rocks available. I have a huge number of very large flat field stones that make up my falls.