I visited Liechtenstein and Friedrichshafen earlier this year and travelled between them a circle via both Lindau and Romanshorn/St.Gallen. For comfort I can say I would always prefer Swiss and Austrian trains to DB Regional trains and definitely over a replacement bus. To Romanshorn you can pay on board the ferry, but there are ticket machines at the port (not sure if you must buy before boarding on any routes).
Regarding prices, I bought a Bodensee ticket which includes trains, buses and the Romanshorn ferry, so didn't need to worry about it, however this does not cover St.Gallen to Buchs or the Bregenz ferry. Buses to Liechtenstein are more frequent from Feldkirch and you can pay in both euros and Swiss francs, or by card at Austrian ticket machines, the fares are different though and I didn't have time to get my head around them.