The thing about March is that it's spring break for everyone in kindergarten through college. I would recommend paying more for a destination that gets you around the crowds. We very much liked the Ritz Carlton in Cancun for this. Fifteen minutes from the airport, we got a package deal that included breakfast and siesta meals, and just never left the hotel for a week.
A friend raves about Turks and Caicos as having a private island feel - no crowds and you can bike or walk to many places.
If you want a shorter trip, we like the Hilton Beach Resort in Ft. Lauderdale. Two hour plane ride, fifteen minutes from the airport. Most of the rooms are suites. The hotel can get pretty busy, but it's huge with indoor and outdoor pools and you could almost never leave. Ft Lauderdale isn't the spring break destination it used to be and the beach is wide open with no crowds. My only complaint is that the food is not great.