fill land/nans with 0s

Author: j-atkins

src/virtualship/instruments/base.py

@@ -222,7 +222,10 @@ def _generate_fieldset(self) -> parcels.FieldSet:
 
             # negate depth and reindex (to suit Parcels XGrid strictly increasing depth convention)
             ds["depth"] = -ds["depth"]
-            ds = ds.reindex(depth=ds["depth"][::-1])  #
+            ds = ds.reindex(depth=ds["depth"][::-1])
+
+            # TODO: update when decision on handling of nans/0s in v4 is made (i.e. https://github.com/Parcels-code/Parcels/issues/2393)
+            ds = ds.fillna(0)
 
             fields = {key: ds[field_var_name]}
             ds_fset = parcels.convert.copernicusmarine_to_sgrid(fields=fields)