There is a myth out there that it's okay to thin acrylic paint with water. Well it's not the best idea. Here's why:
Water is not a binder. Duh, right? Well if you use too much water in your acrylic paint mixture, you will overthin the binder in the paint (polymer) and weaken the paint film, lessening its adhesion to your canvas (or the paint layer beneath it). If you want to thin the paint, use a polymer medium. This will maintain the adhesive qualities of the paint and the paint film will maintain its integrity.
If you want to thin the paint and retard the drying time, try using Acrylic Glazing Liquid (Golden). This will allow you to make a thin glaze and slow down the drying time.