Many have different ideas on the subject, but the instructions on most water-based polyurethane recommend you do sand between coats. Begin with the first coat and let this dry completely. Then take fine-grit sandpaper and sand the surface, which will roughen the surface and make it easier for the next coat to adhere. Try not to remove too much of the coating when sanding, the layers are usually thin, and you could reach the wood underneath. This can easily happen on the first coat. Only use lighter sanding strokes and fine paper to ensure this does not happen.