I have an older West Marine Zodiac, about 11', with an inflatable keel and high pressure floor. Not that that matters, but I have an issue with a leak that I can't find. I've done the soapy water thing, the listening thing, et al, to every square inch to no avail. All valves have been tightened and no bubbles showing. I fill it with air, and the next day the port side tube is soft as a marshmallow. It seems as though it's more than just a slow leak. The air has to be getting out somewhere. Any ideas? Thanks.