1. Waterproof Phone Bag $6 online/$22 on Maui - Having worked on a floating restaurant here on Maui, the amount of phones I have witnessed fall into the water is staggering. Dropping your phone in fresh water allows at least a small chance of bringing it back to life, but recovering a phone from salt water would only benefit you as a skipping stone. There's just no coming back from salt exposure.
2. Phone/Camera Float - $7 online/cant find on Maui - Maybe you have a new phone that is waterproof or a waterproof case? That's great! But it wont do you much good if it sinks more than a couple feet before it leaks from water pressure, or better yet, to the bottom of the ocean. Save the phones of your entire family (especially your kids while boating) by getting them floating phone/camera leashes.
3. Long-sleeved Swim Rash Guard - $12 online/$24 on Maui - Very helpful for adults, but a necessity for kids and toddlers. Maybe you are trying to prevent the sunburn from occurring the first time, but very often I see people purchase them because they have already gotten a bad burn and need to guard themselves from a second.
4. Sea Bands for motion sickness - $6.50 online/$8 on Maui - Many people picture a gentle sail or boat ride when they book with one of the snorkel or sailing charters. Unfortunately, most of the time it gets bumpy out there and especially when venturing into the channel between islands. Save yourself by getting some Sea Bands and/or ginger pills to ease your stomach.