This should be fairly simple, I think, and be a useful way of avoiding an exploit: increase by a factor (either hard-coded or set in simuconf.tab) the cost of raising land when the land to be raised is below the game's current sea level. This would simulate the vastly increased cost of sea-based earthworks, and encourage appropriate use of bridges rather than improbable mid-sea embankments.
+1
I second the suggestion. Although that other idea of harbours using the same mechanic as airports would be very welcome to our ship system.
Third this. Slopes can be easily used as exploit for not using bridge/tunnel.
+inf
Try the nighlty ;)