As "mostly" everyone is telling you, go have fun and don't worry about the tip. We usually give $10 as a couple. On a few rare occasions we've given $20 but the guide worked hard for it. He had personality, was knowledgable about our excursion and we could "clearly" understand him.
Of course you have some people that don't tip at all thinking that the price they've paid is enough but they rely on tips just like a waiter does here in the states.
I hope you have a fantastic time.
Mariner-05/06 & 10/09
Oasis Inaugural Cruise-12/09
Monarch-03/10 & 07/10
Mariner; RSS- 10/12
Australia; Celebrity Solstice 11/27/2016