Starfish
Not sure how you’re doing it, but if I’m looking up Montreux - Interlaken connections, the only City ticket option I get is only for Montreux, there’s none for Interlaken.
And obviously, the Montreux city ticket won’t be valid for anything around Interlaken. You’d need to get the boat tickets separately.
However if staying at an accomodation in Interlaken, you’d obviously get the => Interlaken Pass (guest card) anyway, which is similar to a city ticket… check out the details.