I don't recall any free trial app. As my first comment suggests, we tend to like best the app that we learn best. I find it hard to give the second or third app a fair trial. Which is to say that I don't think a brief exposure/trial will necessarily give you a good evaluation. If you don't want to buy it for $50, see if you can borrow someone else's iPad for a weekend cruise.

On the other hand, replying to a different questions, I like the Navionics charts far better than the NOAA charts, primarily because they are "quilted," meaning that they transition from location to location, and scale to scale, seamlessly. With NOAA charts, you have to load the next chart when you drill in or pan out, or when you reach the boundary of the chart you are currently using. It really makes quite a difference when route planning etc, but so does having small-scale paper charts.