Basically how much you tip (if at all), depends entirely upon your own personal opinion. However when taking this into account, you must consider that the employees on your cruise ship may be getting very little money for the good work that they do, and they may be relying entirely on your generosity to make a living. Employees however should not expect to be tipped, but instead offer a hospitable, high standard of customer service and generally in return - a tip is very much appreciated.
Lots of cruise liners add a suggested amount to your daily on board account. It is up to you to keep it included in your account, or you are able to ask that the purser removes this. If you do choose to add the suggested amount to your on board account, you can rest assured the tip is going towards the staff that need it. This is usually divided amongst your waiter, any assistant waiters, the cabin steward and maitre d'.
Some cruise lines leave an envelope in your cabin at the end of the cruise so you can leave behind an appropriate tipping amount. Also, some cruise liners allow you to prepay the tipping / gratuities amount before you leave. If you are unsure if you will have enough money at the end of the cruise, it may be best to prepay this amount to ensure this cost has been taken care of.
For the most part, the tipping culture is not meant to be uncomfortable. Do whatever fees appropriate for you and talk it over with your guest/partner. If you have had outstanding customer service and you want to give some money - great! If you have had terrible service and you don't think any amount is appropriate - don't.