I was there in April of last year and made enquiries in advance and whilst on the island but with no joy. There are a couple of places in Chaweng and Lamai that you can hire speedboats from, though they are fearsomely expensive and so far as I could ascertain only available with a driver (not self drive). About the only thing available for self drive is jet-skis.
Anyone fancy starting a RIB hire business in paradise?