You can use the Google Maps Geocode API.
If your address is in land, the result_type of the response will be something like "administrative_area". if you are in the sea, the response will be "natural_feature".
Here are two examples:
- The first is the 0,0 point, in the middle the the Atlantic Ocean: http://maps.google.com/maps/api/geocode/xml?address=0,0&sensor=false
- The second is located at 20.012065,-6.82251, in the middle of the Saharan desert: http://maps.google.com/maps/api/geocode/xml?address=20.012065,-6.82251&sensor=false
Edit: Some more examples in response to comments:
- This point is located in the sea claimed by UK, within the 12 miles limit. It shows as "natural_feature": http://maps.google.com/maps/api/geocode/xml?address=50.801541,0.424519&sensor=false
- This point is located in the Lake Superior, in the Thunder Bay. It shows as "natural_feature": http://maps.google.com/maps/api/geocode/xml?address=48.23565,-86.981506&sensor=false
