Just a short update back on this in case anyone would find it useful:
- I arrived five minutes late on my TK flight into FDH due to a late departure at IST
- At FDH, you simply deplane by stairs and walk to passport control. Even though I was seated in a window seat and not entirely up front (10F or so) I managed to overtake all other passengers who disembarked before me by simply walking a bit faster. This probably saved me a lot of time
- Taxis were readily available and within 7 minutes after arrival I was on my way to the port - total journey time something like 10-15 minutes only (costs around 15 EUR)
- I couldn't find the Bregenz boat - so I hopped on the 6.27pm to Romanshorn. You can just board the ferry directly - a ticket inspector will walk across all passengers and ask you to buy a ticket.
- I thoroughly enjoyed my glass of Swiss weizen beer from the boat cafe while watching the beautiful scenery from the ferry!
- In the end I found out the journey could have been done much cheaper. The website of the Bodensee ticket suggests that all rail transport up to St. Margrethen is included in said ticket. So I bought a ferry ticket separately with a Romanshorn-Buchs railway ticket as this was cheaper than a Bodensee ticket (with ferry included) and a separate St. Margrethen to Buchs (SG) ticket. However, maps and information leaflets in Switzerland read that actually the Bodensee ticket covers the railway line all the way up to Buchs and Sargans! So just a Bodensee ticket would have sufficed for both the boat and my train connections.