The question of what (and what not) to bring along in the way of dive gear and accessories is critical for the novice setting out to dive coral reefs for the first time. If you plan to use scuba, bring along everything you own EXCEPT tanks and weights. Almost all operators also supply weight belts, but if you prefer your own padded weight belt, bring it.
For both scuba diving and snorkeling, bring along proper thermal protection. Keep in mind that water temperatures in some coral reef areas (e..g., The Bahamas) during local winter can be in the low-mid 70s, while summer temperatures in most coral reef dive destinations will generally be in the 80s. Regardless of time of year, you should bring full body protection of appropriate thickness for the season. Even in the warmest months, you can encounter stinging plankton in the water column, and areas of exposed skin are vulnerable.