Late March is a great time to go to HH. All activities are in full swing on the island and at the resort. It is a very popular Spring Break destination for families.
Temps will range from upper 60's to low 80's. The ocean will still be cool, but the resort pools are heated.
You'll have a great time. We've been in March many times and enjoy bike riding (there are miles of paved bike trails), walking on the beach, enjoying the resort and exploring the island. There are over 300 restaurants and lots of shopping opportunities (for those so inclined!).
Enjoy!