Here is the tipping info from the 
DCL website:
What about gratuities?
It is customary to give gratuities in recognition of service, which you will most likely be treated to in abundance aboard the ship. As a guideline, may we suggest the following:
Per Guest per cruise 	3-Night 	4-Night 	7-Night
Dining Room Server 	              $12.00 	$16.00 	$28.00
Dining Room Asst. Server 	$9.00 	$12.00 	$21.00
Dining Room Head Server 	$3.00 	$4.00 	$7.00
Stateroom Host/Hostess 	$12.00 	$16.00 	$28.00
Dining Manager 	Your Discretion
Room Service 	Your Discretion
Palo 	Gratuity included
Vista Spa & Salon 	Your Discretion
Babysitting 	Gratuity not expected
Shore Excursions 	Optional; not included in shore excursion price.
For your convenience, a 15% gratuity is automatically added to bar, beverage, wine and deck service tabs. All gratuities can be charged to your room charge.