I like nature trips. Hiking, snorkeling, African safari (twice, when I lived in Europe, and Africa was closer) and now diving as well. The guides, DMs, and other service workers in this industry are typically paid low wages, but they choose this work because of the surroundings. They are typically either poor people native to the region, or people who love nature and accept lower compensation because they love the place. While there are bound to be exceptions, and people who have a bad day from time to time, they usually possess a contagious enthusiasm and a ready smile. I could do any of these activities on my own, but the guides and DMs multiply my enjoyment by their knowledge, experience, and their own joy in what they do, and by saving me some of the back-breaking labor (carrying tanks, or in the case of hiking guides, carrying emergency gear). If these people had not chosen these low-paying professions, my life would be poorer for it as my enjoyment of these activities would be lessened. A tip is both a small increase in their income, and a way of telling them that I really appreciate what they do (I also thank them verbally). I want to encourage them and others like them to continue in their job so that there will always be dedicated people there to help me get the most enjoyment out of my nature trips. These people want to make my trip more enjoyable, because they love the outdoors, or the ocean, and they want others to love it also. A tip is more than added salary. It is a way of telling them they are appreciated, and everyone wants to feel appreciated. They're more likely to stay in their profession if they feel appreciated.
On my first diving trip I didn't know how much was a good tip, and I gave the boat $10 some days and $15 other days (for a two-tank day outing). They made it clear that all tips would be divided equally, regardless of who we handed it to. I like that because it makes it easier for me. If I had a really bad guide/DM I probably would not tip. But I've never had that happen. From now on, I'll be making it $10 per tank, and more for extraordinary service. (Oh, yes, as it was a multi-day trip, and they took care of my equipment between days, I tipped for that also.)