Do I need to buy a free ticket for the boat tour in Interlaken at the ticket office with a Swiss pass? Or do I have to board the ship directly and show my Swiss pass on board?
Assuming the Swiss Travel Pass, you just board the boat. Maybe someone will check, maybe not. I keep a printout of the pass in my little bakpack, but normally I just show the QR code using a graphic of it on my phone. If you use Google Wallet, or Apple Pay, you can load the pass into one of those. But I'd still keep a paper copy. Gut Fahrt!
The boats operate like the trains. You board, and then once the ship is moving the conductor comes and checks tickets (or passes).
If you have a 2nd class pass and want to upgrade to 1st you can do so at the ticket window in the boat.
Thank you very much