I'd like to add a plug for the hard working professional tour guides who conduct the shore excursions from port to port. About a dollar and a half per person is acceptable for a half day tour, two or more for a full day tour, and if you really had a super guide, go wild. It's appreciated, and a gratuity is expected. Seabourn and Clipper have been known to include a tip for the guide. Tour guides have been stiffed by uninformed passengers on Silversea who mistakenly believe that their tips are included, just as they are onboard. Many of the other lines do tip well. I can tell you, the cruise lines become known in every port, and the exceptional tour guides refuse to work for the poor ones. Premier is out of business, so I guess I can name them, but there are others, some named in this message board. So if you're on a cheap cruise, and your tour guide stinks, it just may be because of the cheap passengers who preceded you and have brought a bad name to the line. Think about it; who wants to work in a diner when you could work at the Ritz?
Please note that tipping is considered a custom, not just a discretion. There is a pay scale miles below minimum wage for those shipboard employees who are suposed to work for tips. Tour guides are free lance individuals hired by the day who lack any employment benefits or health insurance, and often find work only in tour season. Tipping is supposed to be a two way street; your service provider treats you well and you reciprocate. So thank you to all you good tippers, and heads up to the uninformed!