The idea is to cut a length of the inner tube with the valve still on it that will run the entire length of the crack, seal the ends, insert the tube into the crack, inflate the inner tube to seal the crack. The rubber is pliable enough to expand into all the nooks and crannies as long as you inflate it with enough pressure. The higher the pressure in the inner tube, the higher the water pressure (or taller the head of water) it will be able to seal against.
I haven't gotten a chance to test this on the porch but here is a little experiment I did to test the concept...
Step 1: Materials
-needle and thread
-multi-tool (always handy)
-solvent based glue
Step 2: The Hole
Step 3: Wash Out the Tube
Step 4: Seal the Ends
It is important to fold the end of the inner tube over as this fold is actually the seal; the stitching is just to keep it fold in place.
Start sewing by pushing the needle through the fold as seen in the first picture. This may be a little tough so keep a thimble or folded up piece of paper at hand; a pair of pliers from the multi-tool also came in handy.
Loop the thread around the side of the fold (not over the top) as you push the needle back through to the first side. As seen in picture two, stop before the needle goes all the way through. Use the trailing end of the thread (the piece thats on the side of the tube w/ the business end of the needle) and wrap it around the needle 3 or more times. Then pull the needle the rest of the way through; This will create a knot and allow you to seal the outer most edge of the fold.
Now proceed across the fold to the other end of the tube, going over and under as seen in picture 3. When you get to the other end repeat the knot you used to start the process.
Step 5: Seal the Crack
Once the tube-patch is in place in your crack, inflate it through the exposed valve to about 15 psi, or as high as you dare. In theory 15 psi is enough to seal a crack at the bottom of a 30 foot pool, assuming that your folds and seams are cut pretty well. In reality I would probably only trust it to seal about 10 ft of water, but the higher the pressure your seals can take the better your tube will protect against leaks.
The patch worked, but only while I kept pumping air into the tube. AKA, my stitching wasn't enough to seal the inner tube at the required pressure. I will have to get a solvent based glue for the real application. I will however still stitch the ends as I don't think the glue alone will hold up to the pressure either.