I did a day and night dive at Kapalua Bay not too long ago. Great marine life given the fact the bay is sheltered. Fairly sizable lava canyons run away from the beach out into the deeper bay containing lots of moray eels and crustaceans. Rented a UW scooter from the onshore dive outfitter for the heck of it, and had a great time flying through the narrow canyons.
Did you also have to rent a guide? Have not heard of any "just" rentals of scooters here in Hawaii for many, many years. Could you give us more contact details?