HAL's policy is as it states - no tipping "required" and they do mean it. You are not required to tip. However, tipping is allowed if someone did an outstanding job for you.
Tipping occurs at the end of the cruise, customary cruise ship tipping is usually about $3 per day p.p. But with HAL you can tip less and it is acceptable.
If you want, you can tip a little at the beginning of the cruise "to insure promptness" which is what TIP stands for!