Hi Dave,
ReefMaster currently interpolates depth, but does not extrapolate. Interpolation can be over distances up to 200m apart, depending on the setting chosen (200m is very high, and can lead to significant inaccuracies - higher values should be used with caution). You should be able to fill holes near the shore by including a shoreline with a set depth. RM will then interpolate from the shoreline to the nearest track-points, assuming these are within the interpolation range.
Having said all that, we have had requests for extrapolation as well as interpolation and we do intend to add this at some point - but I can't give you a firm idea right now of when that will be.
If you have areas wider than 200m that you wish to fill, and have a good idea of the depth in those areas, then there are some other tricks you can use to provide spot sounding data in those areas. Try using a "boundary", configured with the required depth and settings provide-depth=yes, act-as-boundary=no, show-in-map=no. You may only need to provide a couple of points in the empty area to allow the interpolation to "bridge" through the empty area and fill it with interpolated values.
Matt
