IMHO, the problem of terraforming lies not so much with current supplies (although it makes it much easier if these are not scarce) but with the planetary supplies of the materials for water, namely hydrogen and oxygen. Remember, water accumulation is accomplished through various geological and biological processes. One way water is formed/accumulated is through the formation of new plant and animal organisms. Introduce a population capable of thriving without off-world supplies, and the process of water accumulation will begin. This route is a little on the slow side, but it is much more economical than attempting to transport water in from off-world.