When doing my budget, I figure (and could be I am wrong, I have no idea) 15% of the cost of the excursion and then add to it if we had a grand time or if the service was above and beyond. I will say on our last cruise we did two snorkeling excursions where the tour folks told us where the tip jar was and asked for tips. I had already planned on tipping but I got the impression some other folks felt bad that they didn't have any cash with them.
Cab drivers are another story - did I get there safely or are my knuckles white and my knees a knocking? I generally round up - but how far depends on the service and my perceived safety.