Acrylic paint can be applied as thick or thin as you like. You can use it very thin and 'watery', in transparent layers (glazes) like watercolors. Or thick and buttery, full of texture and showing brush marks (impasto). I love both; it depends what mood I'm in and the subject of the painting.
When thinning acrylics, the rule of thumb is not to thin it with more than 50 per cent water otherwise you risk it losing its adhesion properties (ability to stick to the canvas). Use some glazing medium as well as water.
Like so many aspects of painting, there is no "right" or "wrong" answer to the question of how thickly you should apply acrylic paint to a canvas. It's a matter of personal preference.

