Unfortunately these fixes are not viable. These things can reduce the leaking in the short term.

Fiberglass, if made thick enough would work for some period of time, but even that is likely to fail. Saw a large water feature where that was tried many years ago and it was a complete failure. Partly because it was ugly, partly because it still leaked. Plus fiberglass would be very expensive compared to just rebuilding correctly so I don't consider that to be viable.

The problem is the foundation, basically a pile of rocks that is falling apart. Nothing is going to hold that together. Say you injected very crack with epoxy, that would fix it...until it developed more cracks.

The other issue is non-leaking waterfalls are difficult to build even when done correctly. Not hard, not impossible, but it requires some level of detail that many people chose to ignore. Water rarely takes the path we expect. For example water can move up hill in a waterfall.

Tearing down the waterfall and rebuilding is far less work imo and gives you the best chance of success. You can reuse the current rock and you don't have to clean off all the existing mortar as it will blend in or the few bits that stand out can always be cleaned up later if they bug you.

My suggestion is tear it all down. Breaking mortar and concrete is all about litttle taps, not big powerful swing. Vibration breaks mortar. You don't need or want a huge sledgehammer, a small light hammer is fine. A chisel is the main tool, normally called a "cold chisel". You want the smallest tip and longest you can fine even if you have to order online. Something like 1" wide tip and 18" long is good. Long because it can be used as a pry too and it keeps your hands further from the rock which is safer. My best chisel for this is 16" long and comes to a point, I think it's a center punch. And of course a air powered chisel works well too but you need an air compressor. You can buy air hammer chisels and just use by hand.

Safety goggles unless you like grit in you eyes and trips to the eye doctor.

Once removed you want to start with a good foundation. For 7' foot high you really want a poured concrete pad with rebar, like 6" thick and below the frost line. Above the frost line isn't super bad so that's a corner that can be cut. You can do away with the pad and just use concrete block, basically make a pad out of blocks filled with concrete. You do want this all on undisturbed soil, not back fill.

Create the basic shape you want with block keeping in mind the rock veneer will be thick. Underlayment over the block and then EPDM liner. Mortar rock over that.

To build leak free think always assume there will be a leak. Suppose you get done and this thing leaks...how do you find the leak? In most cases it's impossible because the liner is completely covered. The leak is almost always water moving beyond the liner, not a hole. So all we care about is the edge of the liner right? I build so the edge of the liner is exposed. When I'm done you can see the contiguous cut edge of the liner. So there's rocks outside the liner and rocks inside the liner, but at no point do they meet. Eliminates wicking (water running up hill). I test the falls with running water and can see exactly how close water is coming to the liner edge because I know exactly where that edge is. I can make adjustments. When happy I mortar a little rock over the liner edge, or a combination of real and fake rock, just enough to hide. OK if some is exposed because it's so thin a line you just have to break up the pattern a little. If you ever have a leak in the future this thin covering can be removed to check for leaks.

Waterbug:

Thanks so much for taking the time to respond. You clearly have experience in this area and I am grateful for your insights. We live in Tampa Florida, do you know any good pond building guys in this area?

In my heart I know the correct way would be to simply rebuild from scratch.

I don't know of any good pond (Water Garden) building guys any where. I'm sure some must exist somewhere, but I've never run into any. I avoid them like the plague.

Swimming pool builders are on the other hand are very professional and know how to build waterfalls correctly. I would trust any of them. However, that comes at a $$$ cost because they do it right. A downside is most build a specific look, not what I'd call natural. More like what you'd see in Disneyland, pretty fake looking. Some pool builders subcontract the building of falls and similar features to specific people. These guys would only be known in the trade, they don't advertise. But you could try asking at a local mason supply. We'd still probably be talking some serious $$$. Like $500 min.

I think you'll be surprised how easy and fast the current falls can be taken down. And building one is pretty easy too depending on how far you want to go in making it long lasting. Use mortar, any kind, instead of foam or crap like that. Don't have to be a mason.

The downside is although there are a couple of technical structural details that are pretty easy to master the actual look and sound of a falls is pure art. That takes some practice. My first few falls were pretty crappy. But once you see how easy they are to redo you can always try again after awhile.

Waterbug:

Thanks again for your insights. I may try to do it myself. I have never done masonry work but I am sure I could learn.

I stayed away from masonry work for a long time. Built my first 2 above ground ponds with wood because I'd had some carpentry training. I thought the wood pond would go up fast. It didn't. It was surprising difficult and I could see several weaknesses. So on my next above ground pond I tried concrete block. Wow was that easier and about the same cost.

So for the past 15 years I've been a big fan of masonry. There are a lot of technical stuff in masonry, but almost none of it applies to DIY work. For example, when you go buy mortar mix you might notice there's Type N,S,M,O etc., and wonder which one to get. Doesn't matter. Technically you want N, but for this type of work it just doesn't matter. And people will create trouble saying it will leech lime and kill fish but that's just people making up stuff they think is true.