[Edi]

Not for Dom2. That's not doable by mod in any case, you need to make a special map-file for a particular map to make water provinces barren.

The way to do that is to use the province commands for mapmaking to set population in water provinces to zero, set sites there to zero (#killfeatures) and poptypes to completely non-recruitable, meaning poptypes outside the range of existing ones. For every single water province on the map.

Those procedures don't eliminate the defenders in the provinces, but defenders in a province that is otherwise utterly unusable makes for a strong deterrent unless you can use it for a shortcut while going someplace else.