Salt and wind. If you are staying for a long time, you have to worry about salt spray and the damage that will occur over time to your trailer. Wind from the water will damage your awning if you leave it unattended all the time. When we are at the beach, we keep the awning closed unless we need the shade outside while we are there.
Enjoy, camping at the beach is awesome. Relatively cheap, relatively close beach access with all the comforts you bring with yourself. I love it!