i've been using the 18456 extension ring with the megadome and gotten good results at f/5.6 and smaller apertures, both with and without a diopter. not sure if its the ideal extension, might be worth trying the next size up, 18463. call up aquatica and talk to jean, he may have tested the lens.
i've also used the zoom gear for the 12-24mm lens and it works great, its a lot longer than it needs to be, as teh focus and zoom rings are reversed to the 12-24mm but you put the ring on in reverse, oring side on the zoom, and the other bit will turn the focus ring but that doesn't affect focus with the clutch mechanism in autofocus mode.
i'd also suggest calling up reefphoto, and seeing what size extensions they've figured with other housings, like subal. i assume they've played around with it as they sell the lens, i bought mine from them. maybe also ask them to take a measurement as to how far the lens sticks out from their housing to compare it with your aquatica. those guys are really helpful. cheers,
chris