Funny you should mention Cardiff airport, as I've often wondered if there'd ever be a case for a car ferry across the Bristol channel. In my younger days I worked at Taunton Deane MSA and the number of Welsh accents heard was remarkable! Was there ever a ferry historically I wonder?
A car ferry did of course exist between Beachley & Aust - until the Severn Bridge opened in 1966. Cars were also transported on trains from Severn Tunnel Junction.
At one time, a passenger hovercraft operated between Weston & Penarth.
In 2011, a company issued plans for Fast Cats to operate in the Bristol Channel. Their web site has never been removed -even though nothing has come of this proposal.
http://www.severnlink.com
Taunton Deane is just over an hour from Cardiff so it would (& is) common for people from Wales to use these services. (Most Cardiff people do not have a Welsh accent but you would definitely recognise those from the valleys & further west by their accents).
The most viable route for a car ferry would probably be between Swansea & Illfracombe as the Second Severn Crossing and M49 shortened the journey between the south west & Wales. A Fast Cat service - similar to those from Portsmouth to the IOW, may be successful. On the Welsh side, Cardiff - with its excellent shopping area would be the main attraction. Swansea does not have an attractive city centre - though plans exist for improvements.
An hourly direct (all stops) train service exists between Cardiff & Taunton. It is really slow going and takes around 2 hours to do the entire trip - usually in a terrible 150.
The Waverley & Balmoral operate services in the Bristol Channel in summer.
http://www.waverleyexcursions.co.uk/plan-your-trip/bristol-channel/
Plans for a barrage incorporating a tidal power station, have existed for many years. This would run from Lavernock Point (near Penarth) to Brean Down (Weston). A road on top would provide a useful short cut. No mention has been made of a railway & I bet nobody has thought about protecting possible routes as links on either side of the estuary. The latest thinking is Tidal Lagoons - where water is impounded & then flows out through turbines. I really do wonder if this is such a good idea as surely, the water within the lagoon will have suspended mud/sand that will settle out and thus gradually render the thing useless.
http://www.severnestuary.net/sep/resource.html