Answer by Professional Japanese Teacher
It’s good to experience a company dinner if you’re working at a Japanese company. You can say “乾杯（かんぱい）Kanpai!” when giving or joining a toast. Kanpai is made up of two kanjis or Chinese characters. “乾 kan” means “to empty the glass” and “杯 pai” means “sake cup”. So when the two kanjis are placed together, it means “to empty the glass you’re drinking” or “bottoms up!” in English.
However, this does not mean that you have to empty the glass at one go. It’s considered good manners to only take a sip after saying Kanpai! If you can’t drink alcohol, it is still rude not to join in the toast with the rest of your colleagues. What you can do is to accept the first glass of beer offered to you and pretend to take a sip after saying Kanpai! You can also order soft drinks before people make a toast and enjoy Kanpai together with your colleagues with soft drinks.
It is bad luck to make a toast with water in Japan because it means you’re bidding farewell forever. So never offer or join in a toast with water.