sorry if im being a big dum dum but you're trying to make a bridge thats in or under the water that land units can cross? From what i've messed around with I think a mix of simple height adjustment / vertical scaling and setting your CSGs to pathable will allow you to make bridges that go under and above water no problem
Like a bridge with shallow water over it. Everything I tried the CSG went into never never land never to be seen again.
You could try placing it on the water, and editing the .pas to lower it a little. However editing the map in the editor again will probably make the bridge disappear.
mmm its probably not what you're looking for but I had some luck carving out an underwater path that land units can cross using the "desert pit" CSG
