I generally don't like stickers on cars but make an exception on the back window of my truck. My philosophy is that someone that knows skiing and/or boats will recognize it (I notice it on the one other vehicle I see around here with one) and if you don't know what it is I don't really care.
Just a 12" MC sticker I cut the little skiers off of a KD sticker that I had and put them above which makes it a little different and sets me apart a bit from those X boat owners

.