distance problem in GWR

Hello all,

I am about to perform a GWR analysis using measurements collected at sea
onboard a ship. Some of the stations are located at opposite sides of an
island or within adjacent bays... Measuring bird's flight distances between
the stations do not make sense at all those cases. 

Is there any way to presently instruct the R's implementation of GWR that
the distances are to be measured along sea-ways? This would probably
necessitate the concurrent input of a land mask (or polygon).

As I presume the answer of the above question is NO (no mention of it
anywhere in the GWR documentation), I would like to know how difficult
would it be to add that functionality for a r-novice like me. 

thanks
Servet