I was at the Boiler on a Sunday afternoon/evening about a month ago. Very little is open in Germany on Sundays, so it's a day when the baths tend to be really crowded. Boiler is full of boy-next-door types and interesting-looking to hot middle-age men. It's spotlessly clean. There are lockers and showers on the top floor, a bar with multiple beer and wine offerings and food from salads to full meals available to order on the middle floor behind the reception. Downstairs there's a relaxation area, a steam room where a lot of action happens, a large hot tub with an observation gallery in the steam room, saunas at different temperatures, and a downstairs maze with rooms for the taking. When you meet someone, you find an empty room, put your towels down and enjoy yourselves.
It stays open until early hours on Monday, but the steam room shuts down for cleaning around midnight.
If you get hungry before or after, Berlin's most popular felafel truck parks outside.