out of block, similar to a cistern?
Monty!
Hall!
No but I have a drainage canal three blocks from my house that I think I could use… I could swim all the way into town, or bungee myself to the little wooden foot bridge.
Are you talking about in the ground or above ground?
Above wouldn’t work - “block” (concrete masonry unit) walls don’t work in tension. The thing would explode from the outward pressure of the water when you filled it.
What if you had rebar through the block and filled the block with cement?
Not unless the thing happened to be round and the rebar could form a continuous band around the structure (like in a cistern or silo). In that configuration, the band of steel takes all the tensile forces and prevents the structure from blowing out. Per the OP, I assuming we’re talking about something rectangular approximately 25 meters long and 2-3 meters wide, the rebar would only work (maybe) at the corners. For the length of the 25 meter long side, rebar wouldn’t be rigid enough laterally to resist the forces acting outward (and perpendicular to the rebar).
Try filling a cardboard shoe box with sand and then tell me if wrapping wire around it helps any.
By any chance are you an engineer or architect?
Above ground concrete block cistern with no backfill.
Could you please explain the science behind the ‘exploding’ cistern again. 
The prominent concrete block structure on the southeast side of the house in the AutoCAD drawing is a cistern. The next figure shows Randy putting a waterproof plaster coating on our cistern.
http://www.phys.ufl.edu/~liz/cistern1.jpg
http://www.phys.ufl.edu/~liz/cadhouse.jpg
http://www.a-spi.org/tp/tp43.htm
BTW this is a cistern made of used soda bottles, sure it is round but HEY no wire band around it
Architect.
For starters, the structure in your pictures is a heck of a lot smaller than a lap pool. Structurally, it’s essentially all corners which do provide a fair amount of strength. Also, based on the foundation footing shown in the CAD drawing, it appears like it may be at least partially below grade (yes?). If so, the ground itself would be providing some amount of resistance against the outward forces.
Back to the OP, the issue is that something 25 meters long would need to be braced along the length of the 25 meter long walls - especially in the center. Concrete block alone won’t do it - neither will block with rebar. You need some sort of support that will act perpendicular to the wall surface. Rebar IN the wall wouldn’t help in this regard - it’s running in the wrong direction.
You’re also dealing with the worst case scenerio, because you’re trying to maintain a water tight enclosure and ANY movement or cracks will compromise the pool. Back to the shoebox analogy: fill it with sand and the corners will hold together but the sides bulge. The longer the side, the more it bulges. If you’re trying to hold water in with a material that inherantly brittle has joints every 16 inches, then NO bulge is acceptable.
You would have to pour tie beams and columns. Also you would be doing yourself a favor to place it at least partially below grade.
Great ideas buckeye and dicknixon!
I will put it at least partially below grade. Then backfil correct?
Or perhaps easier to just pour the walls but probably quite a bit more.
Size would be just as Buckeye said about 25 meters but just one lane.
Another way to do this above grade would be to effectively shorten the long sides by building butresses along the length of the wall.
For below grade, your soil type and compaction requirements will be a factor, i.e. if your soil type is sandy or loamy, you might need to beef up the structure anyway. I think.
these folks will sell you a kit of 25 yd x by 1 or 2 lane above ground portable pool:
I built my pool out of poured cement, and I don’t think any amount of block would have worked without backfill. My walls are 8 to 12 inches thick with a ton of 1/2 inch rebar throughout. The hard part was building the interior forms. It was like noa’s arc in there. The wood alone cost me 6 grand. Then there is the steel strapping and the anchors into the poured bottom, that was 8 inches thick. In all, I used about 180 yards of concrete for a 75 by 27 ft pool, that is 5ft+ deep throughout. It is about 80,000 gallons of water. I’m sure some engineer here could tell you the stresses that this volume puts on the walls. I guess if you used block that had rebar and was filled correctly, and then you backfilled to the top, then it would be pretty much the same as maybe a 4 inch wall or something. I think that there is some added strenght to having continuous cement poured over rebar, at least that is what I was told…Good luck, it can be fun if the money thing doesn’t bog you down…
http://www.slowtwitch.com/mainheadings/products/camps/xantusia.html
A few pics of the finished product…