Nov 5, 2019 - 8:33 PM
Based on the SBB timetable, it looks like there are two boats running on Lake Thun, alternating directions of travel. The trip is listed at 2h 12m, with 10 stops between Interlaken West & Thun, and the same going in the reverse direction.
I have not been on the Lake Thun boats but the boats I've taken on Lake Lucerne had small restaurants that served an abbreviated menu of "real" food, not just prepackaged food. I never needed to book a trip in advance, just buy a ticket or use the Swiss Travel Pass. There is a difference in price for 1st & second class tickets. 1st class is usually for the upper deck, at least the Lake Lucerne boats I rode.
Returning via boat or train is up to you. The train will get you back in 28 minutes, and costs considerably less if paying for tickets. The STP covers either one.