So how would Reidak give a Makuta a challenge? Makuta of Metru Nui has plasma, sonics, oh, and about 40 other powers.
EDIT: Well, I've gotten info on the Nocturn-breaking-his-island situation.
First, he didn't smash it with his own power. He used some kind of blaster which he grabbed in a fit of temper. And he shot the island at its critical point and caused it to crack in half. The island itself is, or was, smaller than Metru Nui, and had about 3000 of his kind on it. So they'd probably a good amount of tech.
On Voya Nui floating, it had a buoyant substance in the terrain (after all, the bottom section of it was once a ceiling) and that kept it afloat. Mahri Nui sank because lava leaking out of the island hardened underneath it and caused that section to become heavier than water. That's the same lava that formed the cord, and the same lava that created that underwater section of the Voya Nui bay, exactly where Mahri Nui was. So it makes sense, to me at least.
On Voya Nui's lava, that's a secret. ;-)
And finally, ice sinking. Well, there's only one instance of ice sinking, and that was when two Barraki were frozen in it. There isn't really an explanation needed, the ice obviously sank because two human-sized beings made largely of metal were weighing it down.
I apologize if I sounded like bones, but Helden and Ikki asked for geek-speak, so I'm giving it to them. :-P