We will be there for one night on our way back from WDW and on our way to Charleston Place in Charleston. We were there last April, beginning of the month with my parents and 3 teenagers. We were in short sleeves and shorts/capri's for the whole week, but we always bring sweats and sweatshirts for "cooler" days. We spent one full day hanging at the pool in bathing suits and laying on the beach and it was probably the most perfect beach day of my entire year. We did alot of touring of the island, and DF, and DS played golf twice, so we actually could have spent more time at the beach, if we weren't busy with the other things. HH is really great! The resort is beautiful, offers alot for families, beach house in itself is a nice place to hang out and only steps down to the beach. I could spend a whole day dolphin/pelican watching at the beach. Harbourtown is a great place to spend a day shopping, dining and walking around the marina. Went to the top of the lighthouse for a beautiful view!
I think the end of April it will be warm enough to swim and wear spring/summer clothing. However, I am an overpacker so there's always a pair of slacks, a long sleeve blouse, a sweater with me, and sweats with me... just in case.
Have a great time!