Oceans are the most polluted due to all garbage being disposed of there. Currently, there are many concerns regarding pollution and environmental hazards. Each one of us, should be mindful citizens of the world and ensure that we leave the place better than we found it. Therefore, reduce plastic usage as much as possible. Even if you do, ensure you don't leave any packaging and waste behind. Always collect your own garbage and dispose them in the appropriate garbage bins. You'll be setting an example to other tourists as well!
Furthermore, do your research well and ensure you don't pose any threat to any of the marine life. For example, coral reefs are being under attacked by humans and also current weather circumstances. Volunteer to help if there are opportunities available.