It may be that the nature preserve might go beyond the shores. This is the question.  However, I was using parcel data that did appear to be a better shoreline than the original data. Being new to the mapping of nature preserves on existing islands, I am still learning.  I think I have started doing what you said and tagging the existing island as a nature reserve. One question is the nature preserve blots out the island name when rendering after I do this.  But I did see one that was rendered with the island name appearing and the nature preserve name along the side of the boundary.  That was what I was hoping for but maybe I was doing something wrong or being to analytical about how it is dispalyed on the map.