Most luxury cruises either automatically include gratuity on a passenger’s tab, or provide a suggested tip amount for each staff member. Prepaid tipping is also increasingly common on cruise ships, though passengers are usually allowed the option of freely adjusting their tipping percentage at the ship’s service desk. Altogether, the average suggested total gratuity ranges between $10 and $17 per passenger, per day, according to Frommer's.

These days, cruise ship tipping happens behind the scenes, whether it is included in the price of the voyage, or added on as an additional charge. Before you even step onto the ship, all amenities have already been paid for, excluding gratuity and any extras not included in the pre-paid price. On the last day of the cruise you will be handed a bill and need to sign off on the gratuity and additional charges where applicable. You may distribute any extra tips to individual personnel members that you feel went above and beyond, or they can be given out by cruise administration.

Be aware that, for example, even if all dining costs are included in the original cost, you will be handed a bill at the end of the meal. There is no need to add gratuity at that point, as you will do so later. If you choose to leave gratuity right then, it would be additional compensation at your own discretion.
Industry standards are as follows: (according to USA Today and Frommer's)

Room Steward or Stewardess:
$3.50 per day
$6.00 per day for suite guests

Dining Staff:
$5.00 per person, per day

Maitre d’:
$5 per person, per week

Spa Personnel:
If not already included, the suggested gratuity percentage is 15%

Bartender:
Generally a 15% gratuity is automatically stamped onto the price of your drinks

Captain:
It is never suggested you tip the captain of the ship, as he is a well compensated professional, and would likely take offense

(Photo Courtesy of Celebrity Cruises)
(Photo Courtesy of Royal Caribbean Cruises)