You could probably devise some way to "plug" up the intake. I am thinking of a metal or acrylic circle slightly larger than the diameter of the pipe. You could then wrap the disc and the pipe with a good bit of shrink wrap to keep the circle in place when you cut the valve free/unbolt the valve. If you can't hold your breath that long, a 10-20ft section of garden hose attached to something that floats would make a decent snorkel. Don't make it much longer than that otherwise you won't be able to flush out the CO2 with each breath. As long as the disc makes a good seal, a good 8-10 layers of shrink wrap should help keep out the water as well. If this doesn't work and the water starts to come out, DO NOT go back into the water to mess with the plug. You will get sucked into the pipe.
