I will tell what I experienced in 15 years of doing it.
For a dock to dock which is a scheduled tow and not a call out.....the usual was nothing. As often the owner wasnt even there to do it. But if they were, they usually gave $20 for a less than a 2 hr tow, $50 bucks if longer or complicated and sometimes a bit more if it was downright nasty or difficult. Or they were just impressed with the boat handling or info provided throughout the evolution.
Assistance tow boat captains usually dont make squat for their scheduling and overall work routine. Owners of the company might be a bit different. I would say the average waitress where I captained made at least as much or more annually, and if they worked Atlantic City or Resort restaurants, WAYYYY more. Granted they worked more hours, but their schedules were way better as they were not always on call.
Soooooo, tip or dont...... basic easy service I might still do $10....... If you feel the service was above and beyond...let you heart be your guide.
Of course, use that keen mariners eye. If the guy has a ripped up uniform, old boat shoes and bad teeth....think about that tip. If he has a brand new $300 float coat or outerwear, $200 boat shoes, thousands in gold and piercings plus a Rolex....skipping it may not be too bad....