I went to Maui in March and toyed with hitting some kayak surfing. Unless you ship it (read arm and a leg!) you most likely won't find a kayak there. The only kayak shop that I dug up on the internet was in Oahu.
With that said, there are so many incredible things to do in Maui, that I decided to skip the kayaking (which I can do anytime in CO) and do the things that you can only do in Maui. I would highly recommend the Maui revealed guide book for adventures. We snorkeled off lava fields in a place called the aquarium (highly recommend), did some diving which was super cool, went whale watching, which is probably not happening anymore, and did a bunch of cool hikes and chilling at the beach. There are some full on Indiana Jones waterfall hikes to do there. You can also board surf or boogie board if the water is good. In my mind, the kayak thing is just too much of a pain in the ass to try to make happen in Maui and in the end your time might be better spent on other things.
If you want more info you could also search boatertalk. I did a search on hawaii and Maui and there has been a lot of discussion and there were some folks from hawaii who frequent BT.
have a great time!