March is such an uncertain month. You could have mild weather one week and snow the next. And being at the beach, you’ll likely have wind, which always makes the temperature feel a bit cooler. You may want to have a contingency plan. The good thing is you probably won’t have to worry about hotel reservations - you should be able to find a vacancy at that time of year - so if you have a plan A and a plan B, then you can decide based on the weather.

Assateague is a must see, but it's better to go in late April through September. Whenever you do get there, I highly recommend visiting the National Park over the State Park if you are going for more than one or two days.

We went for New Years one year. Freezing but we bundled up and my was still magical.

I visited Assateague island with my family in December a few years ago, and it was great. Yes, it was somewhat cold but there were no crowds. Got to see some of the feral horses as well.