I always throw 5-10 bucks at least....Im in a good mood on the lake, and often these cute college chiks are very friendly!
Especially on the PWC - if they sit down while fueling, smile, and show some leg? - easy $10 baby!
BTW - those stupid tip jars at Dunkin Donuts, etc? I HATE those, in fact I will take change OUT of them if I need it to not break a large bill. I love doing that.