Most things at that time of year will not be booked unless there are multiple ships in port. Also, bear in mind that many places will be closed on the 18th. If anything you want to do is offered on a limited schedule, you might want to book in advance and fit in whatever other stuff you can.
We go around the same time every year as our anniversary is the 17th. It's the best time of year in Aruba, at least in my experience.
Natural Pool, Druif Beach, Eagle Beach, California Lighthouse, Helicopter
When we were there 2 years ago (March 15-29) we didn't make any arrangments ahead of time. The only thing we booked and had to wait for a day to do was the jeep excursion to the natural pool. We just worked other stuff around that which was fine. We didn't do any snorkeling/sailing excursion though, so I'm not sure how far out those get booked.
We're trying to decide what we want to do this trip (March 13-27). Other than snorkeling we feel like we've done it all.