I went back and read the OP. It's about porters, and everyone including me, thought this was the person at the curb putting your bags on the cart at embarkation.
For me, porters are the guys helping get your luggage to your car at the end of the cruise.
Porters on debarkation get $1/bag from me—usually, a minimum of $5 total only because I'm usually out of $1 bills.
These guys are helping me, thus a gratuity,