TIP your boat crew, your DM's, and your Dive Instructors. They bust their butt and make your dive life easier than you can imagine with their long long hours and behind the scenes efforts you'll never see.
tips don't always have to be monetary either. After a long week of diving in a remote island area, and seeing the worn out scuzzy barely functional masks that some of my guides have worn. I give them my mask or other gear/equipment at the end of the trip. You should see their face light up! On a remote Fiji or Philippine island, getting quality dive gear is hard enough if not next to impossible. Being handed a $85 mask for free is most likely almost a months wages for them.
On every trip we go on, before leaving I always go to the bank and get minimum $200 USD in one dollar bills, and $100 in Fives. That way you always have several tucked into your pocket, and able to give to street kids or people who pose for photos.
Getting a case of 5-pack Juicy Fruit Gum and handing out to kids is a great way to make friends. I Belize parents only made $5-10 a day, you think they can afford to buy their kids a simple treat like candy?
I will 100% of the time leave my used flashlight batteries with a divemaster, why the heck should I pack home extra weight of 1/2 spent batteries, when these are like gold to them. Even T-shirts, shorts or Hats, often they love the fact that they now get a shirt that may have a far off lands' sports team on them, or just happy to get some new free clothes.
go to a dollar store before you go, and buy up some balloons, coloring books, or even reading glasses, and donate them to the local church or orphanege. Neosporin Ointment or bandaids too, basic medical supplies are like Gold in many remote areas.
Good Karma pays forward, in so many ways and doesn't have to be $$$
These are really fantastic suggestions! I like the batteries one and medical supplies one especially. Things we take for granted are often a very big deal in poorer places. Great ideas!