Hi David, Kimberly here from landscape solutions for you. Thank you for asking your question for a fast growing tree in the beautiful coastal town of Hilton Head. I have been there a couple of times and wonderful location.
A few specimens come to mind for you.
1. Hollies - they will do well in your zone, but a drawback is they aren't real fast growers. You could get larger established Hollies to plant.
2. Cleyera is another possibility. Great evergreen that will take most soil conditions.
3. Hydrangea Tree is another option. This is a fast grower and will mature to 10 feet.
4. Wax Leaf Ligustrum Privet - evergreen that is a fast grower both height and width.
5. Viburnums - beautiful evergreens that will work well in zone nine.
6. Red Powerpuff - great tropical evergreen
7. Lipstick Hibsicus is a bigger variety of the Hibiscus family.
8. Loropetalum family will also work well as long as you make sure you are getting the larger version. The Chinese Pizazz is a low grower.
The Cypress may work for you with the Leyland or False Cypress.
I hope these few suggestions will help you find just what you are looking for. Let us know what you decide, and if you need a few more suggestions, let me know. Thanks again David for asking the question here at landscape solutions for you. Have a great day and enjoy the ocean!
Kimberly