depends on the rain.. light mist rain it nice. It cools you off while your doing a hike or walk along the beach. Downpour is like nothing you have seen before. Sometimes I had to pull over because I couldn't drive. However I was on BI Hilo area almost all last winter. Wet side and to the most part rain didn't stop doing what you want most the time.
You should have a good trip don't worry about it. Just schedule what you want to do by getting up in am looking at the weather outside - getting on internet for dry side weather (sorry don't know Poipu) and decide what you want to do for the day. Don't make a post it schedule and think you are going to stick to it! Relax and enjoy. It is still warm when it rains... to us mainlanders.
Look into some crocs (shoes) for rain because slippers can get slippery when wet. Who care what they look like you will enjoy them when wet or rocky and eveyone wears them. And you can wear them in the water. Don't do what I did when first came over. Made the mistake of too many clothes, brought leather slippers
no leather. By a beach towel for a gift back home, rent snorkel gear, get your lotions at stores. Forget your makeup you wont need it being in the sun or it will just melt off from the humidity.
Its all good trust you will find what to do that you want when it arrives!!
Aloha and have fun!!