TMM has posted on their website:

WHAT IS THE TYPICAL GRATUITY?

Skippers work hard to provide the best possible service to their clients. If they have done a good job for you, they will appreciate a gratuity at the conclusion of the charter. A rule of thumb is $350-$500 for a one week charter.

A one-day Check-Out Skipper (COS) gratuity would be a percentage of the fee of $175. Twenty-five dollars tip for a COS would be a good rule of thumb. All gratuities are optional.

Horizon Yacht Charters has this posted on their website:

We provide professional skippers and cooks at all our bases. Their fees do not include food and beverages and optional gratuity. Crew gratuities are usually based on 10 to 15% of the charter fee.

Sunsail States:

The average tip for a skipper is 15 to 20 percent of the skipper’s fee. This is, of course, at the client’s discretion.

The Moorings website says:

What should we tip our crew? The quality of service should dictate the gratuity. As a guideline, we suggest an average of 15-20% of the total charter cost for Crewed Yacht Charters, and 15-20% of the daily Skipper fee for Skippered Bareboat Sailing or Power Yacht Charters.

Conch Charters says:

Gratuities are greatly appreciated upon completion of the charter and usually a minimum of $30 a day.

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I stopped looking at this point, but most companies have an FAQ which answers this question. If they don't, they should ... so ask them!

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As you can see, the recommended gratuities are all over the map! I was recently chastised by an unhappy captain because I told a customer that the going rate was about 20% of the captain's fee ... or more provided they were super happy with him.

I was told (by the captain) that it should have been 10 to 15% of the charter fee, which came as news to me!

The best thing to do is to find out (before booking) what the expected captain's gratuity will be. Nobody wants their captain to go off in a huff after a great time out on the water. I think the industry really needs to get this sorted out, as it is definitely causing problems.

When a bareboat skipper is hired, he or she should be told the charter company policy on tipping and they would then have the option to accept the job or not.

So I suggest you ask your broker or the charter company what their policy is and get it in writing.

One of the companies above says one thing on their website ... but in practice, they say something quite different. Their website has likely never been updated. It's easy to miss these things.