Jan 18, 2015 - 3:59 PM
If you're traveling in April, the Spring timetable applies. In Spring there are only a few boat rides a day. The last one from Thun all the way to Interlaken West departs from Thun at 12:40 and arrives in Interlaken at 14:49, and this one is available on Saturdays and Sundays only. There's one later departure at 15.40 (on all days), but this boat only goes as far as Beatenbucht. You could get out there and continue to Interlaken by bus. In any case, you would have to complete your visit to the Rhine falls quite early to combine this with the boat ride to Interlaken.
A boat ride over lake Thun is very much worth it in my opinion, and fun for kids too. If you don't manage to combine it with your Rhine falls visit, then you may want to do it as a separate day trip from Interlaken.
All information about the Swiss Travel Pass, including purchase options, is available at www.myswissalps.com/swisstravelpass.
I hope this helps you out!