Actually all the water comes from you cat. The cat turns your exhaust gas into more non-harmful gases and also water vapor which collects in the pipes and comes out water. The by-products of a internal combustion engine do provide moisture but not water because all water is either burnt on the power stroke or is combine with carbon and makes it a very sooty. As long as the water is coming out clear or a little dirty(if you have debris in the pipe from muddin ) then dont worry. If its really hot and humid you will see more water.