Adding tape to your watercolor painting process gives you more than just a nice crisp edge. It can also add interesting design effects. Whatever tape you choose, make sure it can be easily removed from your paper. I recommend testing this with your favorite medium to make sure that when it is removed it doesn't destroy your painting in the process. I use either delicate surface painters tape or traditional washi tape on my watercolor papers.
Some of the techniques I use in my sketchbooks include simple edge definitions, defining borders, creating distinct blocks of color, and creating geometric designs, like the star shown below.
Paint over and around your tape then peel off slowly at a 45-degree angle to reveal those satisfying crisp lines.