It depends on your needs and the crew's attentiveness and service provided. Most I have been on have a jar and the crew shares tips. I would do a minimum of 10% of the cruise price and more if you felt you had exceptional service. I have seen some tip nothing and others tip very generously, especially if a lot of alcohol is provided. If you happen to be on a smaller boat and the crew is the owner(s), I would still follow the same as I stated above but hand it directly to them as they may not provide a "tip jar".
Hard to arrive at a benchmark as every person has a different take on tipping and how much. It's all personal and each situation will dictate what you feel is right.
You don't think there is a "benchmark" for restaurant tipping? It is odd then that some restaurants actually put 3 different tipping percentages on the bill/check if there is no "benchmark" or guideline.
I am guessing the original poster has perhaps never been on a day cruise before and was looking for a specific suggestion.
I have understood standard as 10% of cost of excursion - with a minimum often below 10% of $10 per person for a one day sail excursion - paid to crew as a group or if only one crew then individually. Never known a crew on day sail boat that did not share tips, even if no tip jar, so that you might hand tip to one crew member.
I'd check with the captain, feel out there thoughts. As for me and our gang when we go out on the Rhino Safari riders, my crew tips our guild a twenty each! We have a ton of fun, our guild gives us free rain to motor where we want!! We are always a private group, not mixed in with cruisers! https://www.rhinotours.com/
Yes, I agree with jazzgal. 20% is about right. Usually a tip jar is passed around, or at the exit when you leave the boat. I would put the tip in there. I would not tip an individual as it might cause problems if the Captain sees that, he might feel he should get a portion of it. I'm sure the crew knows the guidelines of what is their expected portion of the tips, and they probably divvy it up according to those guidelines. You're on vacation, they're not-be generous !!!
So it seems 10-20 % is the number most are putting out there.
We recently chartered a boat for a full day cruise for 4 passengers. It was just us and the captain/owner of the company. We didn’t tip but…it was an 8 hr private day with the owner. As a business owner myself, I didn’t think a tip was necessary. But if there were crew, I may think otherwise.