which gps app can do this?

[arne anka]

why not simply draw a circle with the boat's current position as center  
and the given distance (3nm) as radius and then check if the circle  
contains part of the coast line? the moment, the circle is empty, you're  
out of range.

still no scenario for an usual gps app, but the coast lines should be  
available with almost all data (openstreetmap frinst) and thus easily  
checked.