If you want to paint with watercolors on canvas, I'd suggest that you paint on a watercolor canvas. Watercolor canvases are a fairly new invention, and they are perfect for artists who like to work in watercolors but also like the look and feel of canvas. As you can see below, you can get watercolor canvas in several different forms: stretched canvas, canvas boards, and canvas pads.

Normal canvas, even if it has been gessoed, is generally not absorbent enough to work well with watercolors. The watercolors would lift off too easily, which would make blending or overlaying colors particularly difficult. There's a way to do it, using Golden Absorbent Ground, which you can learn about here.

Now let's talk about watercolor canvas...

Painting on watercolor canvas is a different experience than painting on watercolor paper, so there may be a learning curve as you first start out. For instance, the watercolors will stay wet longer on the watercolor canvas than they will on paper. Plus, even though the watercolor canvas is coated with a gesso that is specially formulated for use with watercolors, the surface is still not as absorbent as watercolor paper.

To get the most out of watercolor canvas, you might want to make your first painting on watercolor canvas an experiment, so you can test out different techniques and take note of how the watercolors react to the canvas. This can save you from making lots of mistakes when you paint a "real" painting on watercolor canvas.

Many artists prefer working on watercolor canvases rather than watercolor paper because it means they can hang the finished painting directly on the wall, without needing to frame it under glass. If you do this, be sure to protect your finished watercolor painting with a spray sealant, such as Krylon Crystal Clear. This will form a protective final coat over your painting, similar to a varnish.