I haven't worked for a cruise line, but I have worked places that pooled tips. Even if they aren't allowed to keep individual tips, the thought is very much appreciated. Knowing you thought enough of their service keeps most in the service industry going and trying harder.
Also, with pooling tips, there usually is some sort of accounting procedure to show who was tipped what amount from the total customer base. so the staff (and managment) is aware that Joe brought in the standard amount for the week, Mary brought in under the standard, and Tim brought in over the standard. If those numbers are consistent from week to week, the peer pressure is going to get intense (no one wants to "carry" anyone else), and management can look at it as well, and make changes if necessary.
If an employee is giving exceptional service, their tips will generally reflect it (with some strange exceptions like having a group that didn't budget for tips). And if an employee is giving less than standard service, their tips will generally reflect it.
I don't know if this is done on any cruise line, but it is possible.