There are differing opinions on this subject, but the labels of most water-based polyurethane products will recommend sanding between coats. Once the first coat has dried fully, you should take some very fine-grit sandpaper and give the surface a little rubbing. This is meant to provide a little bit of traction that helps the next layer to adhere.
The important thing to remember here is that you don’t remove any more material than necessary. A single coat of polyurethane is not very thick, and it doesn’t take a lot of effort to sand through to the bare wood. This danger is especially present when doing the first coating. Use light strokes and fine sandpaper to ensure you don’t go too far.