I should probably mention that I've been working on getting bridges across water into the sandbox ruleset in 2.6, as a side project. There are a couple of engine fixups I need first (to do with integrating cardinal roads) so I haven't got anything to publish yet, but here as a teaser is the help text from my WIP.
Road bridges are large bridges carrying roads across one tile of Lake or Ocean. They only run between edges of tiles (not diagonally). It is not possible to build a bridge more than one tile long, and parallel bridges may not be built on adjacent tiles.
One notable thing is that it's quite common (particularly with hex tilesets) to end up with bridges that don't go straight across a tile, but turn a corner, or have three or four exits; so tilesets will need to take account of this (I think this can be done simply, or elaborately by drawing a wide variety of exciting interchanges -- the bridge is its own extra, not just a road on water). There's nothing really to be done about this; after looking at this Wikipedia article
I don't feel so bad about that.
I've limited the length of bridges over water to one tile by prohibiting a bridge tile next to another bridge tile, and used first_reqs to make them start next to land (and ConnectLand for drawing). This has the side effect of no parallel bridges, which is a bit ugly, although as above they would also look totally weird if allowed.
WRT dirt bridges: I've set Steel as the minimum requirement, so a road bridge can reasonably look modern. I've made them quite expensive (but not as expensive as Transforming; 16 settler-turns -- I didn't expect the Øresund Bridge to cost as much as the Deltawerken, although the difference is smaller than I expected).
With the current engine, I can't enforce that the bridge is between two land tiles. So there's nothing stopping you building piers all around your coast and parking land units on them to 'occupy' the tiles, which is a bit annoying.