I read this on another board and tried it. The results were fantastic. Take your tip money in the form of two dollar bills. The first person we tipped was the porter at the pier who handled our luggage. When we handed him a two dollar bill, he got this huge smile on his face and said, "THANK YOU SO MUCH! My grandkids collect two dollar bills." At that time, another porter overheard our conversation and came over and asked if we had any more, he wanted to give us two one dollar bills for each one of them. We let him do that with one of them but told him we had to save the rest for others we needed to tip. The two dollar bills seemed to bring a smile to the face of everyone we gave one to. Just a suggestion.
Just a thought, and not to burst your bubble, but I would only use them in ports in the US or Caribbean, and as tips to the crew of ships that home port in the US. Having spent a lot of time in Mexico and Costa Rica, I have personal experience with the difficulty of using US currency that is in any way "odd" such as torn bills, bills on which people have written, bills over $20, and, in general, anything which they can't readily identify as genuine US currency. So your waiter or cabin steward might have trouble USING the money if it were out of the ordinary. Just a thought.
That happens in the US too! I went to pay for a sandwich once at an Arby's, and discovered that I didn't have any cash. Since I always carry a couple of $2 bills, I tried to pay with one of those. The kid at the counter not only wouldn't take it, but called his supervisor over and told him that I was trying to pass counterfeit money! I was speechless. The supervisor stared at the bill for a long time and finally asked why he suspected it was counterfeit. "Well...it's a $2 bill!", the clueless teenager said. Needless to say, lunch was free. :-)