Very touchy subject for many of us... on day trips I give 8-10 a tank if it was an enjoyable trip but if attitudes are brought out, nothing to not much is given and a note to maybe find another boat...
Much like restaurants, cabs, barbers, hotels, shoe shine guys/gals, and everything else we seam to be tipping for; I am a big believer in raising the price of the meal, ride, haircut, shine, or what ever to avoid all of this nonsense...! I am a good tipper as I find the manager when the service is exceptional but on the flip side, I have demanded to pay full price when I had a 2 for 1 dinner coupon and the service was so bad I was not going to leave a tip come hell or high water and I did not care that the coupon stated 20% grutity automatically added...! Funny thing, when they asked who my waitress was and I pointed her out all the could say was "Oh" like they already knew...
I just wish tipping would go by the way side... Its not but I sometimes wonder; if I am the only one on the boat from the US and everyone else is from Europe should I still tip???
Tipping is all about your own personal attitude... either you tip and tip well or you don't or tip poorly... Last boat I was on, one of the guys I knew was offering 5.00 total for a two day 5 tank dive... I gave 45... Some will say I was wrong for not giving 50.00 while others will say 25.00 would have been enough...!!!
Personally, I am just tired of working the numbers... if dinner was 46.84 for the two of us, I just add 10 or maybe 15 depending on the service... Hell, I gave a 10.00 tip for a really bad haircut... lol, it was really bad but I thought I did really well... The girl was messing up, almost in tears and I was just biting my tongue, telling her "every thing is fine" hoping to make it out of the chair with enough hair so I could find someone else to straighten it out... true story!!! Lucky I am alot older and still have an appreciation for having hair but back in the day... It would have been ugly!
As to your original question, If I was in the Philippines or another country, I would break it down to 10% of the cost of the trip and split it among them, more if it was earned... But that is just me and while some will agree, others will disagree...