I think that if you book the excursion through the cruise line, the gratuities are already included in the price of the excursion. Of course, you may wish to tip the bus driver a dollar per person in your party ... or a tour guide a buck ... but that's about all I tip when I take an excursion through the cruise line. And, believe me, I see plenty of people who don't bother tipping a dime.
But if I were to take an excursion on my own, then I would tip a bit more generously if the service warranted it.
Blue skies ...