Do you tip a captain, and how much?

A 20% service charge is included in what you are quoted. It is not a tip, and you are not expected to add one.

People do add something after a long day, an early start, or a driver who did something genuinely above the arrangement. Any tip is optional, and it goes to the captain who drove you.

There is no screen to swipe, no suggested percentages, and nobody standing there while you decide. Tell the concierge afterwards, or add it to the ride, and it is handled without a moment at the kerb.