I mean aside from cash. One pax told me he was sorry that he didn't have any cash to tip me but gave me his card (he ran a little jet ski rental place). He said to give him a call the day before and we could go out jet skiing for a while. A few days later I called him up, after riding with him for a while we went back, he hopped on his own jet ski and we went out for a lot longer than I was expecting. Easily one of the best "tips" i've ever received!