Something we've experienced more than once when cruising with family is handing over our baggage to a porter when boarding. The tips are "optional" but we shudder to think what might happen to the luggage on its way to the ship if the tip isn't forked over. IIRC one or two cruise ports in the U.S. have even outlawed tipping for that reason.

We roll our own luggage aboard - the tipping of course is only one reason. If you've ever seen the piles of a luggage on a typical cart during embarkation or disembarkation, you're going to try to avoid it.