1. Buy reef-friendly sunscreen if you’re going to swim in the ocean.
Some sunscreens contain a chemical called oxybenzone. Even small amounts of this chemical can do a lot of harm to coral reefs, so if you’re going to go swimming, try and buy a reef-friendly sunscreen instead. A bunch of them are available at this link.
2. Get a reusable water bottle.
The water restriction in airport security is a pain, but there’s no reason you can’t still bring an empty reusable water bottle and fill it once you’re through security. The less plastic you use, the less plastic ends up in the oceans and in landfills.