Interlaken West station provides a small, basic toilet facility. Perhaps, not the best and cleanest place you can find.
Alternatively, have a coffee somewhere in a street café and use their toilet facilities.
I personally love the Youth hostel at Interlaken Ost station. I go there all the time for breakfast or coffee when I have my shift break in Interlaken. They have a great inexpensive breakfast buffet till 09:30 and a coffee bar, both available to walk-ins, and very clean toilets.
Water is potable in all toilets all over Switzerland, yet not in trains.