Hello mellyndc
See the information on the guest card here; it only grants free travel in Interlaken (in the town):
https://www.interlaken.ch/en/info-service/gaestekarten/interlaken-guest-card
The Libero Day Pass is not valid on the Lake Thun & Lake Brienz boats, only for train & busses.
There is no need to bother with tickets in advance, simply go to the counter in Interlaken and tell the ticket agent from where to where you want to go.
Alternatively, you could purchase a Saver Day Pass far in advance. It starts from CHF 59 and is valid for a day for all public transport in Switzerland including the boats on Lake Thun.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
Or, if you stay longer in the Interlaken area, the Berner Oberland Pass is another great offer, especially if you have the Swiss Half Fare Card in addition.
https://www.berneseoberlandpass.ch/
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
Having provided a lot of ticket advices, I must say that it is always a challenge to suggest best tickets if only fragments of a trip are known, instead of knowing the entire journey. It may lead to wrong advices and wrong (more expensive) ticket suggestions as I miss the complete picture.
Kind regards,
Roland