I agree with Irish Shark, except for the ship's shore excursions. If the guide is just "reciting" from a script we usually tip $5.00 for the two of us. If the guide is actually educating us about their area, answering questions, and being friendly we'll tip $10.00 for the two of us.
That being said, we tip less
to a guide who stands outside the van with his hand out after the tour!
Every once in a while we'll tip the driver $5.00 if they help the guide.
tip the assistant driver, or whoever he is, who sits in the front seat next to the driver. I've never figured out what he does!
I'm sure there will be some disagreement about this, but we're known as "over-tippers"
Have a wonderful cruise,