The cruise ship can help in situations like this by imposing late fees, and they should make them substantial.
This would relieve the caregiver of any hidden hostilities...after all she is going to be paid a good deal extra.
The late parent would know in advance that there are substantial late fees that will be charged to their ship account.
If this caregiver had another client waiting...and now she was late for that job...surely the other client would have also been very angery that her/day or event was ruined because this caregiver was late. If the shoe had been on the other foot, so to speak...how would SailingGal have felt? I mean, if she had a Spa appointment or ??? and this caregiver had been late BECAUSE another parent was late getting back, and now SailingGal's special appointment was canceled or set back because now SHE'S LATE.
Of course, none of this is really a big deal in the great scheme of things and there is really no excuse for rudeness on anyone's part.
I really believe that explaining upfront BEFORE the job starts that there will be an extra charge for lateness will help.