Ah, great thread. Where to start?
1. Boaters who show up with no fenders or lines because "They didn't know they'd need them".
2. Boaters who saved money by buying the small "Tylenol Capsule" sized fenders rather than the big boys.
3. Boaters flying through the no wake zones with no clue (Unless it's 2am

)
4. Boaters who throw anchor way too close to you- the ones you'll be running to the bow to fend off in 3 minutes.
There are many more, but I'll stop here.