It depends on what the guides do for you. I have tipped $1 to $2 per person for a really good guide/bus driver - one who gives island information, etc. (we usually travel as two adults)I did give someone an extra $5 once for going over the top with adjusting the air conditioner, the seats, etc.-he really took good care of us and gave a lot of info. If all they do is drive the bus or boat with no additional info provided, I don't tip.
I wouldn't have tipped the boat drivers for parasailing, as we didn't carry cash and it is just about 15-20 minutes.
