1. Select biodegradable personal-care products like shampoos and soaps whenever possible: Everything you flush or rinse is going into the seas you are sailing.
2. Pick out the absolute least amount of clothing you think you will need for the length of your cruise, and then cut it in half. Really.
3. Ask your live-aboard operator what dive tools are needed in the region you are visiting -- pointer? reef hook? SMB with extra line? -- and then make sure you have the training to be comfortable using them.
4. If your cruise includes stops at local villages, ask what items you can bring to share -- like pens or pencils for schoolchildren -- or what local needs are.
5. Bring a Sharpie or indelible marker, then buy a souvenir T-shirt on the boat. Have the crew and guests sign your shirt for a unique but affordable souvenir and a fun memory.
6. Depackage everything you can before you go - like batteries and meds. The developing world has trash problems of its own, it doesn't need your detritus too.
7. You've heard the one about bringing candy to share with your fellow passengers, but remember to include the crew too: It's a good way to get to know live-aboard staff beyond your divemasters, and intoducing them to new flavors fings out the kid in eveybody.