On our excursions, (GC & Cozumel), we tipped both places. I did notice, we were one of only 2 families that tipped in GC. There were probably 10-12 different families on our excursion and I was a little disheartened by that fact. There were 3 crew members and they split all tips between the three of them.
On our excursion, the team in GC mentioned tipping once and let it go at that. The team in Cozumel however, mentioned multiple times that tipping was appreciated. It got to the point my DH and I would look at each other and smile. The mentality in Cozumel about asking for a tip is very different than in GC. They are not afraid to outright ask in Cozumel! We did buy a photo package on the Cozumel excursion, even though it was outrageous and along with a smaller cash tip, we figured the two balanced out and the team got some $.
One thing I will mention, is the exchange rate. Keep in mind when doing any tipping in either place, the exchange rate is not 1/1. The US dollar in GC is lower and therefore, if you tip what you think is $10, it's really only like $8. (At least when we were there.) Check the exchange rates before you leave so you have an idea what your US dollars is worth so you don't give what you think is a fair amount, but is actually lower than what you thought. I wish they'd put the exchange rate in the Navigator the night before ports...
I was disappointed by those who didn't tip even a little. We tip for services here in the States, why is it any different when you go on vacation? Our cruise was a stretch on our budget as well but excursions, tip included, was part of how we did budget the $ we brought on-board.