Once again,,not an expert here, but I was on the Volendam last August. Once on the ship you are asked to register a charge card at the Front Office (like a hotel front desk). On the last day at sea we were given a bill for all the charges we incurred during the trip. The theory was with bill in hand,,one could let it go and have the charges remain on the credit card or go to the front office to make other arrangements, and that would be the time to take off the 10 dollar charge, which BTW, was per cabin per day,,not per person per day. I choose to go to the front office and pay it off with cash. I was 4th in line, didn't take any time at all.
So you wouldn't actually forget because the bill would be staring you in the face.
I do wonder, though, about the money in the pool theory. Not saying any one on the ship is dishonest, but who would know if the cabin steward was slipped a couple of bucks? But,,heres a good one,,after the first night I left my steward some money on the bed. Later that day he comes running down the hall after me with the money in hand trying to give it back to me. He said "Sir, you shouldn't leave money around in your cabin in the open,,someone could take it. I found this lying on your bed". I explained to him it was his and any money he finds on the bed will be his. I ended up leaving him quite a lot because he was excellant,,did he get to keep it,,who knows?