One of my best purchases last year was ten dollars spent on an iPhone app called Marine & Lakes produced by a company called Navionics. It has many features but for me what I love most is that it displays water depths based on your location. At the shoreline of many of the local lakes I know the water depths from experience but it is very helpful to give buyers exact information and show them the depth changes. Below are some screenshots from the app which also is available for your iPad. There is an Android version too that can be found in Google Play.