From my experience as an instructor / guide
Americans tend to tip by default - it's a cultural / social expectation in the country, almost regardless of quality of service.
There is a certain type of British person who will always tip - and actually it's more often the sort of "working class come good" - people who started out their lives with not a lot of money and did well for themselves. They appreciate a good service, and are happy to reward people for it. I think this is partly because they see us as having something in common - working hard for not much ready cash.
Most Europeans do not tip - it's not an expected social phenomenon, but some do, and they tend to be quite generous when they do. String is right - it often comes through beers rather than cash, but that can be quite a lot of money, actually!

Rich people almost never put their hands in their pockets. This is, I suspect, why they are rich.
I run the risk of opening a whole can of worms I do not wish to eat afterwards here - but here's another thought: Because Americans are renowned for tipping, when they do not, one feels disappointed, as though one has not done well enough. When they do - great! When Europeans do not tip, one does not feel disappointed, because this is expected. When they do - actually that sometimes makes the value - if not the amount - of the tip higher, because it is not so common, and therefore is more of a compliment for a service well provided, if that all makes sense.
For the record, I also do my job to the very highest of my ability regardless of the nationality of the people with whom I am diving. I expect no tips at all, and feel slightly embarrassed about receiving them - but they are received with a great deal of gratitude.
Cheers
C.