I normally don't acknowledge their friendly wave. Some don't realize what they're doing as they've never owned a smaller boat. Others know what they do and it's out of spite. I'll normally shake my head in disgust as they go by, and have been known to offer up a one finger salute if the wake is extremely big. I've never had any problems being very nice on the water, OR turning into Mr. Hyde when prompted. The absolute worst boaters are the Sons that get to take their Daddys boat out for the day. Most of them have needed their butts handed to them their entire life and have been raised to not have a clue.