I've been reluctant to join the thread, but I happen to live on Vancouver Island (near Nanaimo) and I have done some guiding for the local dive shop.
A tip is just that, in my opinion. Something a diver may choose to offer the guide after the dives, on the occasion they had some wonderful diving and felt the guide brought something great to the table.
As for myself, I do guiding to help out the shop. They have a set charge for divers wanting guiding. I get paid by the shop. I am happy with that arrangement, otherwise I can always stop guiding. If I get a tip from a diver, I'm very grateful because it is never expected. My expectation is that I will do my best possible job guiding and showing the diver cool stuff that our oceans have to offer whether I'm tipped or not.
I just enjoy diving.
When I don't feel like guiding, then I don't guide. We have a fair number of really excellent guides at our shop, so I am confident anyone using our shop gets some great dives.