The 30 passenger Candela’s P-12 Shuttle might be a nice, clean, smooth way to get across the Firth, per the City Mobility plan:
If only we were a progressive, creative nation, instead of a bunch of conservative, "moar oil and gas", think-inside-the box-ers...
See: http://citycyclingedinburgh.info/bbpress/topic.php?id=20966&page=17#post-366027