The tall Mex petunias are on the invasives list here, Pam. Even though they still sell them in the box stores, we are not encouraged to plant them. And they absolutely are invasive, popping up everywhere if you don't contain them with a deep concrete edging.
Kate, if you venture out to Florida Native Plants, you will get lots of ideas. Just be sure to tell them you are out on a barrier island. The butterfly area at Selby is another good source of inspiration, and they are right on the bay there.
Have you seen this?
http://www.swfwmd.state.fl.us/...
The fifth entry down is a fantastic resource for Florida Friendly plants and lists salt tolerance for each plant. It's a 110 page download though, so if you can get a hard copy, that is more handy I've found. Our Extension office used to give them out. Check with Manatee (or click "Add to Cart").
What a terrible accident, there. But . . who lets their child ride in a car without a seat belt?