An alternative idea: Maybe grdfill can help you here?
You can replace a specific (elevation) value in your grid (-N) by a constant value (-Acvalue, here probably a small positive value, e.g., 0.3 m). You have to apply this several times to replace all negative elevation values in the area of the Venetian Lagoon (probably something between -8 m and 0 m). You can save the manipulated grid via -G. Finally, you can plot this grid via grdimage as before. As your study area seems to have no other subregion with such small negative elevation values, this should not affect other parts of the grid.