[asheshouse]

I guess that the ground levels for inland areas should be left high so that the game engine knows the correct altitude for engine performance etc.

However what you could do is to use a technique similar to to adding road/rail. In your image editor make a new layer with a single RGB shade to represent 74m level. Then merge layers, subtracting the new layer value from the master layer.

This is just a thought, I havn't tried it so it may not be that easy.