If your guide keeps you longer and gives you more, then 20% is a bargain. On the other hand, if he gives you less and you feel like you got ripped off, don't tip at all. There is no fixed rule. If you tip poor performance, you guarantee that the next client will receive the same. On the other hand, if you tip great performance, ditto.
I will be anxious to here how your excursion went and your impressions about tipping when it is over.
Have a great cruise.