I looked at splines (and even remembered such strange things as inverse Vandermonde matrices...) but then thought that would be overkill and in fact "out of character" for slocum.  I was actually wondering if the wind speeds provided by the models were actual point values or if they represented averages over a grid square (in which case any interpolation would be a questionable "average of averages").  For ECMWF I found a FAQ that confirmed their data was actual grid point data.  I assume the same applies for GFS?.