How to, or how much would it cost, to remove this stone pond

1 Cubic foot of water equals 62.48 pounds - the trick here is calculating the lateral pressure, but doing a little googling I found the following formula:

R =(62.4)(L)(H squared)/2 (R = pounds force)

The force water exerts is equal both sideways as vertically, so using this calculation we can determine the force that will be exerted against the wall at its lowest exposed point by using the long width of the pond as L and 13" (call it 1.1)

62.4*30*(1.1 ^ 2) all over 2 = approximately 1,133 pounds over 1 square foot of lateral force at ground level, which translates to a pretty measly 7.865 pounds per square inch (1,133/12/12). (If my math is right, and I think it is a TAD low because if you imagine a column of water 1 inch square and 30' high that's 360 inches of water which is 360 cubic inches which is about a gallon and a half, and a gallon is 8 pounds (pints) ("a pint's a pound the world around")

Since your cement block is filled with gravel and rebar and you've got 1k lb concrete between the two wall layers - I'm reasonably certain that your wall strength combined is sufficient. However, to be ABSOLUTELY certain, you should consult with a structural engineer who can do all the complex engineering calculations that go into figuring lateral, axial and compressive strength of concrete, etc...

If the popcorn finish is fairly even throughout, and you can afford to lose just a bit of headroom, one method is to skim over it with thin wallboard (3/8 is generally available and 1/4 can be special ordered). This is put up in sheets and screwed to the underlying structure (which needs to be located with a stud finder or the poke-with-an-awl method). Then taping, mudding (filling the joints), sanding the joints and painting. Sounds like a big job, but usually less time consuming than taking down the popcorn surface.