The last boat from Interlaken East Station to Lake Brienz was at 15:07, and when we went there, the boat had just left. The staff taught us to take the bus to Brienz, and then catch the boat at 16:40, and take the boat from Brienz back to Interlaken.
We bought a second-class Swiss pass, which allows free boat rides. There are not many seats in the second-class cabin, but there are more open-air seats. If you are afraid of the cold, you can get on the boat earlier. I sat in the restaurant at first and thought I had to consume, but after sitting for a while, I realized that there was no mandatory requirement.