We go to Maui a couple times a year, stay for 2 weeks minimum. We fit everything in a carry-on bag. If you are bringing more than that, you are bringing too much. Ha!
No, you don't need anything other than a swim suit for the ocean. We were there in February and while windy, the water was still very warm.
BTW, in February, there are a lot of whales. Literally look out at the ocean and within 15 to 30 seconds, you will spot a few at least. We went on a whale watching dinner cruise and I gotta say that was the highlight of any trip I've ever taken on Maui. It was out of Lahaina.
Bring binoculars and take them where ever you go.