rsilvers, how about a fishing application? An application to get the wind and wave forecast from the GOMOOS buoys and current conditions from the GOMOOS & NOAA buoys would be useful.
If that is too easy, an application that shows the sea surface temperatures on a map with coordinates of temperature breaks. Ideally moving a cursor to a temperature point on the map should give longitude and latitude and water temp, and time date of the map.
Navionics is a really good iPhone app. Be sure to get the right region for
you. It shows area lake contour maps, buoy designations, etc. There is a ton of information on the coast like tide and current. Worth every penny.