As a dive instructor it is always nice to be tipped, but you should only tip if you have received good service. In Thailand where I work, anything from $10-$15 is the usual.
On a funny note I once had a guy(American) charter the boat for the day, around $1200, I was his personal guide, and in my opinion did a great job, 2 good dives, lots to see, spent the whole day chatting with him about diving, and he spent talking about how much money he had! At the end of the day the guy gave me the equvilant of a 50cents tip, He told me to go and buy myself a few beers, I gave it back to him and told him "if thats all you can afford, you'd better keep it yourself!"